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

Answer: 
My first exposure to étale cohomology was through Bjorn Poonen's notes [Rational Points on Varieties](http://www-math.mit.edu/~poonen/papers/Qpoints.pdf), 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.
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)).