Hey Andrew --
you know, I don't have an official answer for that question. My personal feeling is that the novels are 'secondary' canon, insofar as they are approved by MGM and so carry a level of authenticity that, say, fanfic doesn't.
However, since it's highly unlikely events depicted in the novels will ever be referenced in the series, and since it's always possible (given that the production company folk don't vet the manuscripts prior to publication) something will be referenced in an upcoming episode that directly contradicts something stated in a novel, I don't think they can safely be called primary canon. What's on screen comes first -- in my book, anyhow.
Of course, since many of us are setting our novels in the earlier seasons, and since those days are behind us now, the chances of them contradicting those events are slim. In that sense, I think the novels are canon, because they come under the umbrella of studio approval. Also, the MGM overseer is very clear that we can only include material that has onscreen factual support, or does not directly undermine what has been recorded onscreen.
It's a bit of a minefield. As a nitpicky canon freak myself, I do believe what I write isn't ultimate canon. That distinction belongs to the writers/producers. But after that, there's the novels. *g*
Hope that helps ...
Karen