^Actually it's a myth that canon means "must be accepted as true." Any long-running canon disregards or contradicts earlier parts of itself all the time, as refinements in a work in progress. For instance, "The Alternative Factor"'s assertions about antimatter and the role of dilithium are contradicted by all other canon (including earlier episodes). And even the guy who wrote VGR: "Threshold" has declared that it never happened. In other franchises, you have things like Dallas dismissing an entire season as just a dream, and various comic-book origin reboots like John Byrne's Spider-Man reboot and Superman: Birthright being ignored and forgotten when they weren't well-received.
So picking and choosing is something the creators of a canon do themselves. It's all made up, after all, so if you're just pretending it happened in the first place, it's just as easy to change your mind and pretend it never happened after all. The idea that canon is some kind of straitjacket imposed on creators, let alone on fans, is just silly.
And of course it's got nothing to do with tie-in novels, which are non-canonical by definition. We try to keep the books consistent with each other, but some inconsistencies are inevitable, and sometimes continuity glitches get ignored or glossed over by later writers. So if readers want to make their own choices about what bits and pieces to ignore, they're free to do that. There's no "must" here. You're reading these stories for recreation, not under duress.
