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

Answer: 
Not a textbook, but a free PDF by J.S. Milne, <http://www.jmilne.org/math/CourseNotes/LEC.pdf>, pretty good IMHO.
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.