I was wondering what material in algebraic geometry is crucial and is a logical step for a serious graduate student in algebraic geometry once they've finished Hartshorne. Good answers could include a list of areas of algebraic geometry or important topics that an algebraic geometer must learn along with good references (i.e. accessible to someone with the background of Hartshorne), preferably in the order he or she should/could learn them. Papers in algebraic geometry tend to draw from so many areas within the field itself that I was wondering what people thought was the best order and way of acquiring that material.

3$\begingroup$ I think that birational geometry is such a topic. Concerning the references, KollarMori "Birational Geometry of Algebraic Varieties" may be one of the best. $\endgroup$ – Henri Apr 13 '11 at 7:58

3$\begingroup$ ncatlab.org/nlab/show/books+in+algebraic+geometry $\endgroup$ – Zoran Skoda Apr 13 '11 at 18:36

12$\begingroup$ One difficulty with this question is that the answers, taken together, are inevitably going to give far more (good) suggestions than one person could reasonably go through during a single graduate student career. $\endgroup$ – Charles Staats Apr 13 '11 at 20:19
'Intersection Theory' by Fulton
Every algebraic geometer needs to know at least the basics of intersection theory. Fulton's book is the standard reference and serves both as a textbook and a reference.
'Principles of Algebraic Geometry' by Griffiths and Harris
This is because Hartshorne does not really talk about complex geometry, Hodge theory or more classical algebraic geometry. It might also be good to see the classical approach to the theory developed in chapters 4 and 5 in Hartshorne which of course existed way before sheaf cohomology and schemes.
EGA by Grothendieck and Dieudonné.
This is if you want more of the Hartshorne style algebraic geometry. I would not say it is essential to read the entire EGA, but since it is the standard reference, it is at least worth getting to know it.
Here is a few more suggestions for more specialized subjects:
Birational geometry (KollarMori or Matsuki). Toric varieties (Fulton). Hodge theory (Voisin). Arithmetic geometry (CornellSilverman). Abelian varieties (Mumford). Deformation theory (Hartshorne). Moduli spaces (Mukai).
After Hartshorne, you could start specializing. Find something that seems interesting to you  you'll pick up a lot of new algebraic geometry even though you are studying a specific subtopic. After finishing Hartshorne's book you should be able to read these books without too much trouble.
Dear anonymous, Mumford wrote a short book Lectures on Curves on an Algebraic Surface which, according to the preface, was written for the reader you have in mind (although at the time Hartshorne's book didn't exist yet). The book corresponds to oral lectures and the sections ( called Lecture $n$) are essentially the notes that had been distributed in class after the lectures, which makes for easy to digest little units.
The book contains the construction of the Picard scheme of a surface and the Hilbert scheme of curves on that surface. Lectures 3 to 10 (out of 27) are recollections of the general theory of schemes, with very interesting insights on the functor of points aspect. For example Mumford describes $\mathbb P^n(S)$ in terms of invertible sheaves on the scheme $S$ and their sections, he explains how to describe the Zariski tangent space of a functor defined on schemes even if the functor is not representable, etc.
The actual goal of the booklet is to prove a theorem of completeness of a characteristic linear system on a surface. The theorem was proved in characteristic zero analytically by Poincaré in 1910 but algebraically in all characteristics only in the 1960's by Grothendieck through systematic use of nilpotent elements. But as in all good books, the road is at least as interesting as the final destination, and much can be learned even if the book is not read to the end.

1$\begingroup$ Dear Georges, I came to this question to add this recommendation, and am very glad to find that you've already posted it! Best wishes, Matt $\endgroup$ – Emerton Apr 14 '11 at 6:22

1$\begingroup$ Dear Matt, I am very happy about this identity of recommendation. $\endgroup$ – Georges Elencwajg Apr 14 '11 at 11:04
Lazarsfeld's book ``Positivity in Algebraic geometry'' contains a wealth of important material and is masterfully written. Anyone doing algebraic geometry today will greatly benefit from being familiar with the contents of this book.
Edit: here is a blog post by Burt Totaro on the importance of this topic/book:
http://burttotaro.wordpress.com/2011/01/11/whyyoushouldcareaboutpositivity/
Perhaps the first advice I could give is to ask your advisor or algebraic geometers at the university at which you are located, if you are a student, they might already have areas/problems in mind for you to work on and so can give you the best advice relative to those problems.
A couple books which have not yet been mentioned (some of which I wish I had gone through more carefully):
HigherDimensional Algebraic Geometry, by Olivier Debarre.
This is a nice somewhat more informal introduction that covers many of the topics in KollarMori with I would say more examples. It also covers some of the material in "rational curves" .
Moduli of Curves, by Harris and Morrison
A standard introduction / reference on the topic (which is again heavily studied).
Rational curves on algebraic varieties, by János Kollár.
The study of algebraic varieties by studying their rational curves is a major area of investigation in algebraic geometry. This book is fairly technical but contains a lot of information.
Hodge Theory and Complex Algebraic Geometry I: Volume 1 & 2, by Claire Voisin
Hodge Theory is an important tool and field of study as well.
Mumford's three part series Tata Lectures on Theta is well worth reading. (I wish I had read them already...I could sure use the information they contain.)
Added: Or really, close your eyes and pick a book by Fulton, Hartshorne, Kollar, Mumford, Silverman....If you get the book by Hartshorne that you've already read, pick again. Otherwise, whatever you picked will be a fine choice.

$\begingroup$ wonderful advice, maybe add manin, grothendick,mukai, ...., but the idea is to choose an author you admire. $\endgroup$ – roy smith Apr 15 '11 at 2:32
I've been told that Néron models by Bosch, Lütkebohmert, and Raynaud is a nice book to read if you want to get better acquainted with techniques in arithmetic geometry.

$\begingroup$ Yes, very nice. Especially nice if you want to learn about Neron models of course  but seriously, the exposition is of such uniformly high quality that I it is an excellent reference even for things not (directly) related to the somewhat technical topic of its title. $\endgroup$ – Pete L. Clark Apr 14 '11 at 1:37

1$\begingroup$ P.S.: Were it not for modesty issues, NeronRaynaud models would arguably be a better title. $\endgroup$ – Pete L. Clark Apr 14 '11 at 1:37
As others have said, after Hartshorne you could branch into too many different things, whose relative importance is subjective. Here is my take on what could constitute a good postHartshorne curriculum:
Intersection theory
 Intersection theory, William Fulton.
Classical foundational book, beautifully written.
 3264 and all that, David Eisenbud and Joe Harris.
This one actually tells you how to compute Chow rings in a multitude of situations. But it also contains a wealth of useful information apart from how to compute Chow rings, such as geometry of Grassmanians, Chern classes, Hilbert schemes and a toolbox for RiemannRoch.
Complex Algebraic Geometry
 Complex Geometry, An Introduction, Daniel Huybrechts
This concise book covers nicely the foundational material of complexanalytic approach (which people used to learn from chapters 0 and 1 of GriffithHarris).
 Hodge theory and complex complex algebraic geometry, I and II, Claire Voisin.
It took me time to develop love for this book. Probably because it strives to reach modern techniques as fast as possible, making it hard for someone just entering the world of Hodge theory.
Derived categories of coherent sheaves
 FourierMukai Transforms in Algebraic Geometry, Daniel Huybrechts
Just as all other of Huybrechts' books, this one is a true gem. The book teaches you how not to be scared of $D^b (Coh (X))$.
Geometry of special varieties.
For special varieties (curves, various types of surfaces, special three and fourfolds, etc.) people have accumulated a large amount of more adhoc methods. At the basic level, contemplating the geometry of simplest such varieties provides an excellent playground to truly absorb the power of abstract machinery from Hartshorne. Further on, while doing research you will probably need a very good knowledge of the geometry of your particular variety at hand, so you cannot go wrong with investing some time to learn this material. I will mention here only the basics:
Curves:
 Chapter 4 of Hartshorne or
 Chapter 19 of Ravi Vakil's notes
Provide the absolute minimum here.
Surfaces:
 Chapter 5 of Hartshorne,
 Complex algebraic surfaces, Arnaud Beauville
 Algebraic surfaces and holomorphic vector bundles, Robert Friedman
The first two items are standard sources, while R. Friedman's book is an interesting synthesis of classical geometry of surfaces and analysis of coherent sheaves on them, beautifully written.
 Lectures on K3 surfaces, Daniel Huybrechts
K3 surfaces is truly an amazing class of varieties whose geometry is studied through a multitude of different techniques, and this book does an excellent job showing those multiple facets of research in the area.
 The geometry of cubic hypersurfaces, Daniel Huybrechts
These notes are still work in progress, but as the previous item these notes talk about an amazing variety of techniques: moduli spaces, Hodge theoretic methods, etc. There is also a discussion of higher dimensional cases of three and fourfolds.
The rest is more idiosyncratic since it is the field I am interested in 
Moduli spaces of sheaves.
 The Geometry of Moduli Spaces of Sheaves, D. Huybrechts and M. Lehn
A systematic treatment of foundations of moduli of semistable sheaves. The book discusses an incredible amount of general techniques used in this area.
 Lectures on Vector Bundles, J.Le Potier.
Not the best read with respect to foundations (for that see the previous item in the list), but fully explains the beststudied case of moduli of sheaves on $\mathbb{P}^2$, which still serves as a beacon for research in this area.
 Fundamental Algebraic Geometry. Grothendieck's FGA explained, B. Fantechi et al.
An introduction into some more advanced fundamental techniques useful for moduli problems: descent theory, Hilbert and Quot schemes, elementary deformation theory and Picard scheme.
 Deformation theory, R. Hartshorne
A nice concise account of algebraic approach to deformation theory, with a lot of examples.
If you are interested in complex manifolds I would recommend Complex Geometry: an Introduction by Huybrechts.
I also think the Toric Varieties by Cox, Little, and Schenck is an excellent introduction to many advanced topics in algebraic geometry. Plus you get to learn a bunch of combinatorics in the process!
Q.Liu's "Algebraic geometry and arithmetic curves" is a good reference, if you plan to lean on the more arithmetic side.
Again, a good idea would be to discuss the matter with your advisor(s), since there is a rich variety of fields within the topic (puns intended)  or ask a more precise question.
I'm far from having read all of Hartshorne, but if I did I would study Compact Complex Surfaces, by Barth, Peters, Van de Ven. Also Geometric Invariant Theory would be a nice topic (I know about the book by Mumford, are there other good books on this topic?).
Ah, I forgot! How about derived categories? Someone suggested that for this topic a good reference is the book by Hartshorne Residues and duality. I had a look at some notes by Caldararu on the arxiv, "Derived categories of sheaves: a skimming", they seem well written.

7$\begingroup$ I'm not familiar with he book by Bart, et. al., but if you're interested in Compact Complex Surfaces, then the book by Beauville it's quite excellent. He covers the Enriques classification in a very downtoearth nice way, given that one has the tools from Hartshorne. $\endgroup$ – anonymous Apr 13 '11 at 9:36

1$\begingroup$ For GIT, there is also Dolgachev's "Lectures on Invariant theory." $\endgroup$ – Phillip Williams Apr 13 '11 at 16:00

4$\begingroup$ For derived categories, there is a nice book by Huybrechts called FourierMukai Transforms in Algebraic Geometry. $\endgroup$ – Chuck Hague Apr 13 '11 at 16:23

3
Geometry of Algebraic curves, by Arbarello, Cornalba, Griffiths, Harris, 2 volumes; Complex abelian varieties, by Lange and Birkenhake; M. Artin, Lectures on deformations of singularities.
The basic idea common to all these suggestions is to use the foundational material from Hartshorne to investigate some more specialized topics, like curves, surfaces, abelian varieties, moduli spaces, singularities, or some more general techniques like intersection theory with applications to general Riemann Roch theorems, and vanishing theorems with applications to classification questions, plus arithmetic and analytic questions.
edit: In answer to a request in a comment below, this is an attempt to give some guidance to reading ACGH, Geometry of Algebraic Curves. I apologize if this bumps it back to the top.
A main theme in ACGH is to give, for a curve C, a model for the abel map C^d>>W(d) in Pic^d(C), which is a resolution of singularities of the Brill Noether variety W(d), in terms of the natural “kernel resolution” of the locus of singular matrices in the space of all matrices of a given size. One is especially interested in the case d = g1, where g = genus(C).
I.e. in the space say of square nxn matrices, one has the discriminant locus D of singular ones, defined by the determinant. This locus is to be a local model for the theta divisor W(g1) of the Jacobian of C = Pic^(g1)(C). It has a natural resolution in the product space P^(n1) x Mat(nxn), consisting of the set of those pairs R = {([v],M) where v is in the kernel of M}. The second projection map R>D is to be a local model for the abel map C^(g1)>W(g1) resolving the theta divisor.
To set this up locally, choose a line bundle L0 in Pic^(g1)(C), and a general divisor E of degree equal to h^0(L0), so that the map H^0(L0)>H^0(L0(E)) is an isomorphism. Then H^1(L0(E)) = 0 and the map H^0(L0(E))>H^0(L0(E)E) in the exact sequence 0>H^0(L0)>H^0(L0(E))>H^0(L0(E)E)>H^1(L0)>0 is the zero map.
For line bundles L near L0, we map L to an nxn matrix, with n = h^0(L(E)) = h^0(L0(E)), for the corresponding restriction map r(L): H^0(L(E))>H^0(L(E)E). This maps a neighborhood of L in Pic^(g1)(C) to matrix space, sending L0 to the zero matrix, and sending a nbhd of L0 in W(g1) to a nbhd of zero in the discriminant locus D. Via this map, the abel resolution C^(g1)>>W(g1) is locally the pullback of the natural kernel resolution R>>D of the discriminant, in particular the fibers of the abel map are the linear series L, i.e. the projectivized kernel P(H^0(L)) of the map r(L).
To make all this work, one must study rank loci for matrices, as well as methods of representing the family of linear maps H^0(L(E))>H^0(L(E)E), for L in an open set of Pic^(g1)(C). This takes a lot of heavy foundational machinery in acgh, chapters 2 and 4, constructing Poincare’ bundles and so on. But this is where they are going. See pp.834, 1767. Chapter VI has some nice geometry, and chaps V, VII state nice results.

1$\begingroup$ Try Riemann's paper on Abelian functions, and AndreottiMayers paper on abelian integrals, and George Kempf's book on abelian integrals. $\endgroup$ – roy smith Apr 23 '11 at 5:13

$\begingroup$ Dear @roysmith: sorry to join the party too late. I am trying to read Vol. 1 of Geometry of Algebraic Curves and find it a little hard to penetrate, even after a reasonable understanding of Harthorne (in my notsohigh standard). I was wondering if you have any suggestions regarding how to make reading Vol. 1 smoother. Many thanks in advance! $\endgroup$ – karl Jun 14 '16 at 2:05

1$\begingroup$ maybe ACGH is too complex analytic for you after Hartshorne. Have you tried Griffiths' lectures from China on algebraic curves? that uses a lot of complex analysis to warm up on. Or maybe if your outlook is more algebraic, George Kempf's book on Abelian Integrals would be nice. for specific advice on reading ACGH, I suggest sampling it, working problems, choosing nice subsections, like the torelli theorem, instead of plowing through everything. there is a huge amount of material already in chapter 1, not to mention the exercises there. just pick stuff that looks fun. $\endgroup$ – roy smith Jun 16 '16 at 20:03

1$\begingroup$ I have edited my answer above to give some hints on reading ACGH. I again highly recommend Kempf. $\endgroup$ – roy smith Jun 16 '16 at 21:45

$\begingroup$ thank you very much for your advice and efforts to help me, I appreciate it! I will need time to absorb what you wrote here, and will take a look at Griffiths' and Kempf's books. I aim to understand both analytical and algebraic sides of the subject (plus arithmetic once I understand the geometry better, which may take a long time to achieve), so it's great to know many good sources! Thanks again $\endgroup$ – karl Jun 17 '16 at 1:42