Question: What is the best textbook (or book) for studying Etale cohomology?

Answer: 
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)
Here are some extra references:

* [Amazeen, Étale and Pro-Étale Fundamental Groups](http://page.mi.fu-berlin.de/lei/finalversion%20(1).pdf);
* [Belmans, Grothendieck Topologies and Étale Cohomology](https://pbelmans.ncag.info/notes/etale-cohomology.pdf);
* [Bergström--Rydh, Étale Cohomology Spring 2016](https://people.kth.se/~dary/etale-cohomology/);
* [Conrad (?), Étale Cohomology](https://www.math.ru.nl/~bmoonen/Seminars/EtCohConrad.pdf);
* [Hajj Chehade, Sheaf Cohomology on Sites and the Leray Spectral Sequence, Chapter
3](http://www.math.leidenuniv.nl/scripties/hajj.pdf);
* [Klingler, Étale Cohomology and the Weil Conjectures](https://webusers.imj-prg.fr/~bruno.klingler/cours/Weil.pdf);
* [Kunkel, Étale Fundamental Group: An Exposition](http://www.math.harvard.edu/theses/senior/kunkel/kunkel.pdf);
* [Laskar, Étale Cohomology](http://algant.eu/documents/theses/laskar.pdf);
* [Puttick, Galois Groups and the Étale Fundamental Group](https://webusers.imj-prg.fr/~jean-francois.dat/enseignement/memoires/M1AlexPuttick.pdf);
* [Robinson, Étale Cohomology](https://www.the-paper-trail.org/wp-content/uploads/2014/03/EC-Essay-1.pdf);
* [Sarlin, The Étale Fundamental Group, Étale Homotopy and Anabelian
Geometry](http://kth.diva-portal.org/smash/get/diva2:1165816/FULLTEXT01.pdf);
* [Szamuely, Galois Groups and Fundamental Groups, Chapter 5](https://www.cambridge.org/br/academic/subjects/mathematics/algebra/galois-groups-and-fundamental-groups-1);
* [The Stacks Project Authors, Étale Cohomology](https://stacks.math.columbia.edu/download/etale-cohomology.pdf);
* [Yang, Fundamental Groups of Schemes](https://www.math.u-bordeaux.fr/ALGANT/documents/theses/yang.pdf);
* [Zarabara, Étale Cohomology over $\mathrm{Spec}(k)$](http://algant.eu/documents/theses/zarabara.pdf).