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.
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.