According to George Lucas himself - and bear in mind, with Lucasfilm now owned by Disney, all of this is subject to changes - everything in a film screenplay or novelisation is considered canon; unless it contradicts something on-screen. This includes situations where he is responsible for both the film and the novelisation. There are contradictions that Lucas has never directly fixed, however, particularly regarding differences between the original films and later revisions, such as the infamous Han shot first incident.
G-canon is George Lucas canon: Considered absolute canon, it includes Episodes I–VI (the most recently released versions) and the upcoming Episodes VII–IX feature films, the animated film, and any statements by George Lucas (including unpublished production notes from him or his production department that are never seen by the public). Elements originating with Lucas in the scripts, filmed deleted scenes, movie novelizations, reference books, radio plays, and other primary sources are also G-canon when not in contradiction with the released films. G-canon overrides the lower levels of canon when there is a contradiction.
I'll offer a few examples. In the novelisation of Revenge of the Sith, written by Matt Stover (but with Lucas's input, as was the case with all film novelisations), the newly-christened Darth Vader and his master, Sidious, sit down and have a polite discussion about the plans for their new Empire, and the best way to eliminate the Jedi. There was not enough time to include this scene in the film, so it was cut, but as there is nothing on-screen which contradicts it, it is considered canon.
In the novelisation to A New Hope, however, there is a scene where General Moradmin Bast speaks to Grand Moff Tarkin:
Bast: Sir, I've analysed their attack plan and there is a danger. Shall I have your shuttle standing by?
Tarkin: Evacuate? In our moment of triumph? I think you overestimate their chances.
This scene appears in the film. There is a difference, however, in what happens next. In the novelisation, Bast, having correctly deduced that the Rebels are capable of destroying the Death Star, sprints from the bridge/ command centre, presumably to take a shuttle for himself. In the film, on the other hand, Bast can be seen in the background, operation a console just seconds before the Death Star explodes. Since the text in the novelisation directly contradicts something that appears on-screen, the film takes precedence.
There are many different rules regarding Star Wars canon, which you can find here. As I said, all of this is now subject to change given the franchise's new ownership.