In reply to Gothick, myself as well I think, consider what's canon to the show to be ONLY the events aired during 1966 - 1971. Despite the official sanction from DCP, I don't consider the audios canon because not all the people involved with the show are involved with the dramas, and I don't mean just the actors. The recent books were officially sanctioned as well, (I believe) but not many people consider them canon, despite Lara Parker penning two of them.
There's canon to the 91 series, canon to fanfiction, canon to the movies, etc. That's jsut my point of view.