Well, I see the prequels and the SE's as canon contradicting itself, rather than of things not being canon, if you see what I mean. You can (under my rules) choose which conflicting item to count as fact, but you can't disregard both of them. The original theatrical release is, in this analogy, the UK first edition alluded to above.

What's not canon by any stretch includes the screenplays, the novelizations, the novels, the comic books, the merchandise, any interviews given by or on behalf of George Lucas, the spiral notebooks, etc., etc.