It's almost a cliche' to even mention this, one of the greatest RPG books of all time. The Player's Handbook!
Hey, wait a minute...something's not right here. I don't even recognize this book! Where did it come from?
Ok, ok. Here it is. The TRUE Player's Handbook!
Words fail to describe the awesomeness of this masterpiece. Gygaxian rambling, I mean prose. Ability tables that have modifiers all over the place. But what made this book really special was the shear ambition it encompassed and the flavorful character it possessed. Full color hard cover, 128 pages, profusely illustrated was unheard of in it's day for the burgeoning RPG market. Instead of me telling you about it, look for yourself.