As others have mentioned, my "favorite" book often changes depending on what campaign I am playing in and/or running, and on what sort of character I am playing and/or thinking about playing.
But two books that struck my fancy are Heroes of High Favor: Dwarves and HoHF: Half-Orcs. Durbin took a core inovation of 3e, the favored class, and ran with it. Reading the books, over and over, made me truly think about all the options opened up by the third edition of the game I love.