Primary Evidence: Canon
The STAR WARS films are the only primary reference. With the exception of only a few minor points, they are indisputable. This is not a merely personal opinion; it is the explicit policy of the Continuity and Production Editors at Lucasfilm. They are interviewed in STAR WARS Insider #23:
What's 'gospel' and what isn't?
'Gospel,' or canon as we refer to it, includes the screenplays, the films, the radio dramas and the novelisations. These works spin out of George Lucas' original stories, the rest are written by other writers. However, between us, we've read everything, and much of it is taken into account in the overall continuity. The entire catalog of published works comprises a vast history -- with many off-shoots, variations and tangents -- like any other well-developed mythology.
Therefore the films and their adaptations are canon, meaning that they take precedence in essentially all matters. Any other form of unfilmed STAR WARS fiction may be official, meaning that it is subordinate to canon and is required to be consistent with other official works. Issues relating to these secondary sources are described below. Works which do not have the blessing of Lucasfilm Ltd (which is not the same entity as LucasArts) are unofficial and cannot be given consideration.
The above continuity policy has been reaffirmed very recently in the introductory pages of the STAR WARS Encyclopedia by Stephen J. Sansweet. His terminology includes absolute canon for canon, and quasi-canon for the secondary and lower official material. He avoids the use of the official non-canon term more commonly used by fans: apocrypha. This may have something to do with the religious pejorative undertones that some readers perceive. However this site uses the more common terms for the sake of brevity. The terms are well understood by most online commentators on science fiction and fantasy literature, and no religious connotations are intended.
