The Canon of Doctor Who

This week I finished reading The Doctor Trap, by Simon Messingham. It's one of the tie-in novels for Doctor Who, which is one of my favourite TV shows, and I got it for my birthday, and I knew it would be a fairly undemanding sort of read, so it seemed like a good choice of reading material. And that was pretty much exactly what it was. There's this fairly crazy guy on a planet full of robots, who's brought a club of people who hunt rare species together to hunt the Doctor. Which in itself is kind of a neat premise, but then they felt the need to overcomplicate it. The Doctor had a double running around, who was all but indistinguishable from the real Doctor, and at one point a whole other double might have turned up, but I was never sure because it just got too convoluted. The writing was reasonably serviceable, though it occasionally had moments that sounded like something I'd have written at age 12. So it was an entertaining book, but it was by no means great literature.

All of which leaves me wondering something. Tie-in novels for TV shows are obviously just another bit of marketable merchandise, and I guess they don't feel the need to recruit good writers if that's all it is. The good writers get to write actual episodes for the show--like Neil Gaiman, who's written an episode for next season. But despite the general (though not entirely universal) mediocrity of these novels, we read them anyway because it's another glimpse of the Doctor. We always want more of his story, and tie-in novels can provide it. Further, because they've got an official stamp of approval, they get to be considered more or less part of Doctor Who canon.
I've read Doctor Who fanfiction that is better than most of these novels. I know fanfiction gets mocked a lot, and I know a lot of fanfiction deserves it, but some of it genuinely explores possibilities in storytelling and character development that the show never has time for, and some of it is written by good writers. I've read fanfiction that I thought was better than the original material. And in a way, Doctor Who tie-in novels are essentially fanfiction.

Do we perceive them as more legitimate because they're in print? Because they're tied to the BBC? They're not objectively better. And with a universe as long-lasting and multimedia as Doctor Who, which first aired in 1963 and is the longest running sci-fi show, with books and radio plays attached, enormous numbers of writers and producers have contributed to build the canon. There's no ultimate authority about what rules Doctor Who is going to follow. In a way, this is perhaps the one fandom in which the lines between fanfiction and canon have become most blurred. So we have to question how we think about what is true in the world of Doctor Who, and how we value one contribution to the world over another.