Best Springer mathematics books I received an email from zbMATH today to the effect that, as a result of their going open access in January 2021 (which is awesome), they will no longer be able to offer their reviewer discount for Springer books since their distribution contract with Springer is being terminated. They advise that all reviewers use any remaining reviewer credits before January 2021, when the credits will expire.

Edit: As David White suggests in his answer below, some community members may find it preferable to donate their extra credits to mathematicians and departments in places where there is a need for books. One possible way to do this is provided by zbMATH and outlined at the bottom of their guide for reviewers as pointed out by Najib Idrissi in the comments on David's answer, allowing reviewers to donate spare credits to the EMS Committee for Developing Countries, however I have been unable to follow the link provided in the guide to see how promptly they use these donated credits (it won't load for me).
Although I've decided to follow Davids suggestion, I'm leaving up the rest of the question since others have contributed to it.

I thought it would be valuable to collect together some community recommendations to spend reviewer credits on before January 2021 since many community members here are likely reviewers for zbMATH as well.

What Springer book(s) would you recommend spending spare reviewer credits on?

I'm a fan of mathematics in general so any recommendations are welcome, but if I had to choose specific topics I'd love a good reference book on algebraic geometry, homotopy theory, combinatorics or synthetic differential geometry.

Second edit: Since the community decided to close the question, I'm going to leave it up and closed until January then delete it; there's no evident way to make the topic less subjective, and it will be irrelevant after the credits are expired. Until then, this post can still hopefully serve as a resource for people who want to donate or spend their expiring credits.
 A: Polynomials and Polynomial Inequalities, Peter Borwein & Tamás Erdélyi (GTM 161). There's something for everyone in there.
A: Of my modest library, Neukirch's Algebraic Number Theory is easily my favorite. It's approach to class field theory avoids cohomology, so a student without a heavy algebra background can use it as a second course in algebraic number theory. I appreciate the third chapter as a down-to-Earth glimpse of Arakelov theory as well. Certain sections, exercises, and remarks hint at deep connections between number theory and algebraic K-theory. And of course Neukirch's style and organization is simply delightful.
A: I am by no means an expert but I really like Introduction to Number Theory by Hua Loo Keng (L.-K. Hua) because of its broad coverage of not just the standard topics in a generic number theory text, but also including chapters such as Schnirelmann density and geometry of numbers.
A: I, too, received the email that the reviewer credits would be expiring. Perhaps this would be a good time to set up a system by which reviewer credits could be transferred to folks with a larger need? I really don't need any more books, but I can imagine that there may be mathematicians and departments in developing countries where these credits might be put to good use. If others agree, and think a large number of reviewers might be willing to give away their credits to those with greater need, maybe we could brainstorm (in chat?) about how to make this happen.
A: M. Aigner, G. Ziegler, Proofs from THE BOOK.
This is a book that every mathematician should have. From the reviews:
"Martin Aigner and Günter Ziegler succeeded admirably in putting together a broad collection of theorems and their proofs that would undoubtedly be in the Book of Erdös. The theorems are so fundamental, their proofs so elegant and the remaining open questions so intriguing that every mathematician, regardless of speciality, can benefit from reading this book. ... "
A: Bott and Tu's "Differential Forms in Algebraic Topology" is probably the best-written math textbook I've come across, and the material covered there is substiantially different from that in, say, Hatcher or Spanier.
A: Although they are basic, I think the books by Lee on topological manifolds, smooth manifolds, and Riemannian manifolds should be on such a list if we are talking about which books are most generally used by the most people.
They continue to be extremely useful to countless mathematics students and people who are self-studying mathematics including physicists who need to learn more about manifolds.
A: Anything by Serre.
I particularly admire Trees. One has to get past the formalism, but then one gets an incredibly concise exposition of many powerful theorems. It feels like every word was carefully weighed (even when read in translation). For instance an immediate corollary of the result that a group is free if and only if it acts freely on a tree is that subgroups of free groups are free.
Another remarkable Serre book is Linear representations of finite groups, which has three parts, at wildly different levels. The final part is still an excellent reference for the decomposition map relating group representations in zero and prime characteristic.
A: Since you mention algebraic geometry, here are some Springer books that I use frequently:

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*Algebraic Geometry: Hartshorne, Robin

*Geometric Invariant Theory: Mumford, David, Fogarty, John, Kirwan,  Frances

*Geometry of Algebraic Curves Volume I: Arbarello, E., Cornalba, M., Griffiths, P., Harris, J.D.

*Positivity in Algebraic Geometry I & II: Lazarsfeld, R.K.

A: The question depends so much in your field of interest that it is impossible to give an exhausting answer. Some of my favorites in various fields are the classics

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*Nicolas Bourbaki, General Topology

*Glen E. Bredon, Topology and Geometry

*Shui-Nee Chow and Jack K. Hale, Methods of Bifurcation Theory

*Klaus Deimling, Nonlinear Analysis

*Albrecht Dold, Lectures on Algebraic Topology

*Herbert Federer, Geometric Measure Theory

*Morris W. Hirsch, Differential Topology

*Thomas J. Jech, Set Theory

*Tosio Kato, Perturbation Theory for Linear Operators

*Mark A. Krasnoselskij and Petr P. Zabrejko, Geometrical Methods of Nonlinear Analysis

*Joram Lindenstrauss and Lior Tzafriri, Classical Banach Spaces (2 volumes)

*Jacques-Louis Lions and Enrico Magenes, Non-Homogeneous Boundary Value Problems and Applications

*George W. Whitehead, Elements of Homotopy Theory

*Eberhard Zeidler, Nonlinear Functional Analysis and Applications (several volumes)

Then there are of course a lot of the Lecture Notes and also some very good books in German.
