My feeling is that canon is in the eye of the beholder. Each 'creative force' behind the show has its own particular canon. When you're watching Original Series, canon is what makes sense within the original series. When you're watching TNG, canon is what makes sense within TNG. When you're reading a novel, canon is what makes sense within the novel.
I refuse to admit the Star Wars prequels as canon. Why? Because they do not fit within the image of the franchise in my mind created by the originals. In my Star Wars, the Jedi are an 'Ancient, forgotten religion'.
The Star Trek novels. If a particular novel fits within your mental image of Star Trek, let it be canon.