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

Answer: 
Not a textbook, but a free PDF by J.S. Milne, <http://www.jmilne.org/math/CourseNotes/LEC.pdf>, pretty good IMHO.
In the web page of [Uwe Jannsen](http://www.mathematik.uni-regensburg.de/Jannsen/) there are great lecture notes of (étale cohomology) courses. In particular:

Sommersemester 2015: [Étale Kohomologie](http://www.mathematik.uni-regensburg.de/Jannsen/home/UebungSS15/seminar-Dateien/Etale-gesamt.pdf) ([Eng](http://www.mathematik.uni-regensburg.de/Jannsen/Etale-gesamt-eng.pdf)).