Everything but very old material that didn't concern itself with maintaining continuity is canon. Its the people who ignore the books that is helping people like Dave Filoni to contradict the books and get away with it. This is in turn ruining the experience for those of us who love the books as much as the films. For example, TPM novelization had a lot of input from Lucas on the backstory of the Sith. Video games are unique in that only their story is canon, not gameplay mechanics.