For me it was Sorcerer. Not picking up the book and reading, but actually playing it.
It's obscure and still in print - do I get triple bonus points?
For me it was Feng Shui. I first played the game with one of the game's creators at a convention in SE Washington and I absolutely fell in love. Great fun and so completely flexible.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.