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

Answer: 
I would highly recommend [these notes](http://www.math.purdue.edu/~dvb/preprints/etale.pdf) by Donu Arapura for a good overview of etale cohomology, as well as [this short paper](http://www.tomsutherland.postgrad.shef.ac.uk/etale.pdf) by Tom Sutherland for an even quicker overview.
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)).