Regarding "fan fiction" and licensed "tie-in" fiction: It's not an issue of quality, just a matter of fish versus fowl. They're two different things, created under very different circumstances. Fan-fic is done for fun and not subject to any sort of editorial constraints and/or standards; if a writer wants Kirk to have sex with Santa Claus, more power to them. Or, less facetiously, if they want to kill off Chekov or invent an entire future history in which the Romulans conquer the Federation, they're free to do so.
The authorized stuff is done in conjunction with the licensor and intended for a wider, more general audience. Outlines, proposals, manuscripts, cover art, etc. are all approved by the CBS. The books aren't "canon," in that the onscreen shows are under no obligation to pay attention to them, but the books must be be consistent with the onscreen continuity as it exists at that time.
Again, no value judgments here. Just two very different animals.