What canon comes down to, at base, is "the part of this story and related edifice that I accept as Real and True, at least for my purposes at the moment." And that's different for every person. I think most people would agree that a story itself takes precedence over anything else, such as cover illustrations, author interviews, tie-in computer games, or what-have-you... but then you get into things like DC or Marvel comics, which have "canon" that stretches back eighty years and contradicts and retcons itself left, right, and center. At which point people generally give up and say, "My canon is So-and-so's run on Title A, What's-his-face's run on Title B, and generic background from Decade Y on Titles C, D, and E, plus some ideas I nabbed from Cartoon Version R and Live-Action Movie H." If you tried to insist that ALL published issues were canon and must be reconciled with each other into a single functional timeline and comprehensible character arcs, you would go stark raving mad in short order.

(I would love to see a "canon is GOD" Lewis-ite try to do that, actually. I think it could be a useful learning experience.)