I've been working through The Jazz Theory Book -- as mentioned by pretty much everyone else -- for a little while, and it's really solid so far.
I've got The Jazz Piano Book, some Aebersold book on jazz ear training, The Real Book, a Berklee book on jazz piano voicings, and uhhh.. some other book on the way for x-mas. I'll let you know what I think of those once I've sat down with them for a while.
In the meantime, spend some time working on 7th chords (major, minor, dominant, half-diminished, fully diminished, and melodic minor), common progressions (ii-V-I, ii-V, V-I, V of V, I-vi-ii-V, etc.), and all the modes. Improvise, too. Do everything in every key. Jazz is hard D: