The two references from my comment above, now with links!
- Milne, James S. Étale cohomology. Princeton Mathematical Series, 33. Princeton University Press, Princeton, N.J., 1980. xiii+323 pp. MR0559531 You can get another set of notes on étale cohomology from his web page: «in comparison with my book, the emphasis is on heuristic arguments rather than formal proofs and on varieties rather than schemes».
- Tamme, Günter. Introduction to étale cohomology. Translated from the German by Manfred Kolster. Universitext. Springer-Verlag, Berlin, 1994. x+186 pp. MR1317816
