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

Answer: 
I'll complement the list of well known books on the subject by some freely available documents, which I find user-friendly. 

[Here](http://stacks.math.columbia.edu/download/etale-cohomology.pdf) 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](https://dspace.ist.utl.pt/bitstream/2295/686086/1/tese.pdf)  on the subject . 

Evan Jenkins's [notes](http://www.math.uchicago.edu/~ejenkins/etale.html) of a seminar on étale cohomology (click on the pdf icons).

The [arXiv notes](http://arxiv.org/PS_cache/arxiv/pdf/1101/1101.0683v1.pdf)  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](http://www.math.u-psud.fr/~illusie/Grothendieck_etale.pdf) (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.
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)).