You guys have given good advise about the important references for those who want to dive deep in the jazz language. Mark Levine's books are indeed very complete works about many forms of jazz. Let me add some more references that focuse on perhaps the easiest (or the most familiar) jazz idioma, based in tonal harmony (present in most of the jazz standards). These books are helping me quite a bit, and the first one is particularly original:

- The New Guide to Harmony with LEGO bricks, by Conrad Cork
- The Harmonic Language of Jazz Standards, by Marc Sabatella

They are both sold online by the authors. Conrad is a brit and his book comes with a very useful "after-sales support". You can meet Marc Sabatella in some newsgroups, like rec.music.makers.jazz .

For those specifically interested in blues, I've never found a better book than this one:

- Improvising Blues Piano, by Tim Richards [Schott Educational Publications]

-- José.