A person who has never read so much as the introduction to Storm Front can own and play DFRPG. They have all the RAW there is to have. There is no more RAW beyond that.
That's a fair cop: it would be a bad precedent to establish that playing DFRPG
requires reading the entire series. Reading the series is a great source of inspiration, but also creative constraint, and it wouldn't really be fair to expect everyone to tack it into the price of admission for this game. Fair enough.
If a question comes up which has an answer in canon, but not in the rules, I think it is fair to differentiate between RAW feedback and canon feedback. And, of course, every group reserves the right to do what they want at their own table.
I feel that it is also a good point to distinguish between what the DFRPG rules allow, what the rules forbid, and what the rules are silent on. And, as previously, every group reserves the right to do what they want at their own table.
I would like to add a fourth designation: what the rules recommend. And I would like it to be given some weight, because even when it isn't outright forbidding or allowing a particular thing, it is still in the book and intended to inform how the game is played.