Oh for...
It is the OFFICIAL stated and explicit policy of the Continuity and Production Editors at Lucasfilm that ONLY the films may be considered canon (or 'gospel). The best anything else can hope to be is official. The difference? Offical just means GL approved. Canon means actually real in GLs universe, and not just a story set within.
Line of command of officiality is: Film, Screenplay, Novelization, Radio. The EU books don';t even register (though they should probably register as underneath novelisation)- but the point is, they are not AUTOMATCiALLY lies, it's just they have no canon status UNTIL they appear in the movie. Just because something is in the books does not mean it CANNOT be right, nor does it mean it does not exist. But it means it is not NECESSARILY right. Only stuff in the films get that privilege. We CANNOT debate stuff that is seen in the films- it is right, and that is an end to it (with the exception of continuity errots like the Imperial Rank Badges in ROTJ... this sort of stuff is also handled by the Continuity and Production Department). But we CAN debate stuff said by the books, and say it is wrong, because it has no more right to be correct than anything that any one of us says. Anything else- quality of the books, what charatcers/ships in the books turn up in the films, even the effect the books have on the Star Wars Universe, is totally irrelevant to that point!
Now, I tend to quote what I beleive to me correct interpretation of what we see on screen- or else I invite discussion on something that, as yet, has no definitive right or wrong. As for my interpretaions, assuming I have made no errors, they cannot be argued with, because the films are ALWAYS right! That is not a personal opinion, it is the truth. E-Mail Lucasfilm if you don't believe me, or read one of the interviews with the department.
In any case- why does it matter? When examined up close, all this stuff about canonicity is really just a technical point dealing with what is and isn't right on the screen, and a little about executive control for various reasons. It in no way impinges on the quality of the EU produce, or it's place in the hearts of fans. When I say EU stuff doesn't count in an argument, I'm not saying that EU is rubbish, I am merely being factual. If the facts were different I would argue otherwise. That's all! I'm really not trying to be hostile!