Ah, the truly wonderful books!
My Three Little Books, OD&D, recieved on Labor Day Weekend, 1975 -- they got me out of wargaming and into the wide, wonderful world of rpgs that I have played ever since!
RuneQuest, 2nd ed -- got me away from D&D until 3e came out. The notion that you don't need classes, that skills are important, and that even a "high level" character can die to a single blow is fantastic.
Ars Magica, 1st-5th editions -- yep, all five. Love this game to tears.
Paranoia, 2nd ed -- come on, it still is a great fun read! Stay alert! Trust No one! Keep your laser handy!
Blue Planet, 2nd ed -- just mind-boggling, the sheer number of possibilities in it! I love this!
Book of the Righteous -- probably the single finest supplement to 3.x D20; even the core books did not do as much for my imagination!
There are a lot of other good ones out there, but these are the core, the best of the best.