The Dark Knight novel is suppose to give us a little insight to what happened to him.
We know what happened to Heath Ledger, RIP, who played the Joker in the last Dark Knight movie. What about the character? We can only assume so much. Apparently The Dark Knight Rises: The Official Movie Novelization is suppose to tell us what happened.
I haven't read this yet, but it is my next read after I am done with my current book. I need more Batman!