Very related to ReadingRoom/TuringMachines
P, NP, and NP-Complete
Has good explanation of Cook's Thm in secn ?, 3-CNF-SAT in secn ?
Hopcroft, Motwani, Ullman
Lewis and Papadimitriou
Wikipedia: NP-complete - The basics. Formal definition, some example problems, references to more complexity classes.
An Introduction to Computational Complexity - Highly readable. Starts by explaining big-Oh, then moves on to P and NP.
Part C of the above goes into more depth in P and NP, including some basic proof techniques.
Clay: P vs NP - a very short, very sweet summary. (Incidently, this is the site for the P vs NP Millenium Problem, a $1M prize problem run by Clay)
Going a bit further:
http://www.nada.kth.se/~viggo/problemlist/compendium.html A bunch of NP optimization problems
Google directory: Complexity Theory - Lots of links we haven't had time to triage yet.
The Complexity Zoo - lots of complexity classes
Cook's Thm(pdf) - proof notes. May be a bit over our heads.