Dawnforge Campaign Setting, by FFG - because I love the idea, however cliched it may be, of a world just starting its Golden Age, leaving all sorts of room for players to make their mark on the world, without too many pesky uber-NPCs to get in the way. Plus, all sorts of cool new variety, whether in races, classes, or background, keeps things fresh and interesting.
My choice would have to be Van Richten's guides from Ravenloft, those little brown book-thingies. They're the ones that got my imagination started with D&D (I didn't start so early) and hold enormous amounts of information on the roleplaying of the undead/lycanthropes/golems/etc... as well as interesting power ideas for them that aren't too hard to convert.
I own the Monsternomicon and its my favorite RPG book not only because it is well written and the art is awesome... but also because I helped write it and won an ENnie for that work along with all the other Privateer Wordsmiths (for those at the last Ennies ceremony I was "that Privateer Guy" ;-)
To enter your chance to win a book for contest #8, simply post to this thread with the name of an RPG book you own that you enjoy AND WHY. This can be something new, old, or Non-d20.
The End (d20 version) is an RPG book I enjoy very much. Aside from the game mechanics, it's got fantastic "flavor text" throughout. Though I usually dislike the fluff and want the crunch, this is a book I find myself reading over and over again.
And I'm having the most fun running an online game of this than I am playing in or running any other game.
I was to the point where building a character for a game was just an exercise in dullness. Now I have new choices, and an interesting system to go along with it.
Complete Book of Eldritch Might. It's a bunch of stuff that I had in PDF, but didn't use, and I didn't want to buy all three of the hard copies. This book solves both problems. Plus, it's got the AU conversion notes for all the book's spells and PrCs in an appendix. That alone was worth it to me.
I really enjoy Manual of the Planes, simply because it tends to get my creative juices flowing whenever I look through it. That's all I ever ask for in a book.
Heroes of High Favor: Half-Orcs because it does one thing, and does it right. Same goes of the other HOHF books, but so far this one is my favorite. And as much as I like the feel of a hefty tome, something just feels right holding this little book that is so chock full of ideas.