Textbook for Etale Cohomology What is the best textbook (or book) for studying Etale cohomology?
 A: My first exposure to étale cohomology was through Bjorn Poonen's notes Rational Points on Varieties, Ch. 6. Not all of the big theorems are mentioned there, but it provides a great introduction to those who have had no previous dealings with the subject.
A: I would highly recommend these notes by Donu Arapura for a good overview of etale cohomology, as well as this short paper by Tom Sutherland for an even quicker overview.
A: Here are some extra references:


*

*Amazeen, Étale and Pro-Étale Fundamental Groups;

*Belmans, Grothendieck Topologies and Étale Cohomology;

*Bergström--Rydh, Étale Cohomology Spring 2016;

*Conrad (?), Étale Cohomology;

*Hajj Chehade, Sheaf Cohomology on Sites and the Leray Spectral Sequence, Chapter
3;

*Klingler, Étale Cohomology and the Weil Conjectures;

*Kunkel, Étale Fundamental Group: An Exposition;

*Laskar, Étale Cohomology;

*Puttick, Galois Groups and the Étale Fundamental Group;

*Robinson, Étale Cohomology;

*Sarlin, The Étale Fundamental Group, Étale Homotopy and Anabelian
Geometry;

*Szamuely, Galois Groups and Fundamental Groups, Chapter 5;

*The Stacks Project Authors, Étale Cohomology;

*Yang, Fundamental Groups of Schemes;

*Zarabara, Étale Cohomology over $\mathrm{Spec}(k)$.

A: I'll complement the list of well known books on the subject by some freely available documents, which I find user-friendly. 
Here are  great lecture notes , from a course that de Jong (of Stacks Project fame) gave in 2009.   
Edgar José Martins Dias Costa's short dissertation  on the subject .  
Evan Jenkins's notes of a seminar on étale cohomology (click on the pdf icons).
The arXiv notes   of a mini-course given by a fine expositor, Antoine Ducros, which also cover analytical aspects of étale cohomology (used for Berkovich spaces).  
And finally a historic survey (in French unfortunately) on the genesis and successes of étale cohomology.
 It was  written  by Illusie, one of Grothendieck's most brilliant students, who acknowledges the help he received in his reminiscences from luminaries such as  Serre and Deligne. 
A: In the web page of Uwe Jannsen there are great lecture notes of (étale cohomology) courses. In particular:
Sommersemester 2015: Étale Kohomologie (Eng).
A: Not a textbook, but a free PDF by J.S. Milne, http://www.jmilne.org/math/CourseNotes/LEC.pdf, pretty good IMHO.
A: Lei Fu, Étale Cohomology Theory is also nice and has not been mentioned yet.
And the lecture notes of Alexander Schmidt: http://theorics.yichuanshen.de/etale-kohomologie/ (unfortunately in German)
