Why, RPG Design Patterns, of course. It described a language for reasoning about tabletop RPGs, and evaluating their design in terms of how each mechanic helps achieve certain desired effects. And hey, cute diagrams!
Seriously, this book single-handedly changed the way I view game mechanics, and its echoes can probably be seen in my pet rule system to this day, many years after I first read it. Can't think of anything better to recommend.