I'll complement the list of well known books on the subject by some freely available documents, which I find user-friendly.
Here 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 on the subject .
Evan Jenkins's notes of a seminar on étale cohomology (click on the pdf icons).
And, last but not least, the
The arXiv notes of a mini-course given by a fine expositor, Antoine Ducros, which also cover analytical aspects of étale cohomology (used for Berkovich spacesspaces).
And finally a historic survey (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.