[BugsBunny]

Not counting "Small Stakes Hold'em" (which I suspect will crack the list):

Theory of Poker - David Sklansky
Hold'em Poker for Advnaced Players - Sklansky/Malmuth
Real Poker II: The Play of Hands - Roy Cooke
Improve Your Poker - Bob Ciaffone
Pot Limit and No-Limit Poker - Ciaffone/Reuben

I only included 1 "strategy" type book. There's a few other's (Middle Limit Holdem for sure) that could go on the list but, to a certain extent, 1 strategy book is enough. The good ones say a lot of the same things, albeit it in different ways. And there are some differences in approach and strategy/tactics, but not enough different to put more than one on the list. Reading the other books listed will help improve your game in different ways by broadening your perspective.

I also left out beginner type books (WLLH and ITH for example)

Poker Essays I,II,III I couldn't decide between them so left them all out. And they don't all belong in a top 5 (unless you count all 3 as 1).

The PL/NL book is there because you didn't specify limit hold'em only. If the game was restricted to limit I'd probably replace that with Inside the Poker Mind.

If you read and apply everything in the 5 books I listed you'll be fine.