[Loci]

For your absolute, no nothing, run of the mill stranger on the street, killer poker. The theory is light, but gets the point of playing better cards across in a manner that a beginner will listen.

novice-supersystem has pretty pictures that make you feel warm and special inside. more importantly, read ITH. Helmuth's first book can be inspirational in that "i'm a whiny millionaire" type of a way...

progressing players- caro and touch on sklansky, malmuth. Tj cloutier's book is a personal favorite.

advanced players- the sklansky collection. Haven't picked up SS2 or harringtons yet.
Cheers
Ez