First of all, I really don't like the term "canon," because of the religious connotation; I don't see Star Trek as a religion. I prefer the phrase "Official Continuity."
The theory I have come to accept (which is my own & of course not binding on anyone else) is that anything produced or published by, or under license from Paramount or CBS as Star Trek fiction is part of the Official Continuity, unless & until it is directly contradicted by an on-screen fact or reference in one of the TV series or films. This would include the animated series, the novels, comics, etc., all of which were officially licensed products. I know it's a pretty broad definition, but I don't think it's unreasonable. If it has the "seal of approval" of the copyright owner, it should be accepted as part of the official universe.
There was a time when I thought only those episodes or films that were approved by Gene Roddenberry were part of the Official Continuity, but I came to believe that he was a bit too exclusive about it. I know it was his creation, but he also acknowledged that many others made it what it was. It has been enhanced by so many great writers, artists, etc., that it seems wrong to dismiss any of them as "not really part of Star Trek."
. . . just an old-school Trekker getting by in a newfangled galaxy.