^None of the novels are canon at all; the term refers solely to the shows and films. The original core material is canon, derivative works from other sources are apocrypha. Many fans use the word "canon" when they're actually talking about continuity. I think the latter is what you're referring to here -- the fact that a lot of early novels had their own idiosyncratic takes on the Trek universe and don't quite fit with the others.
Christopher, I stand corrected. Yes I did mean continuity
rather than canon
. It is interesting to see how many of the early novels definitely seem to happen in a kind of parallel universe to the films and shows.
And then there's Shadow Lord which isn't so much an idiosyncratic takes as it is buhwaaaahh?
Haven't gotten to that one yet! I'll have to see what you mean when I do.