I think it is a good idea, but just playing devil's advocate for a moment, why is a pdf document, fixed at a point in time, better than this forum? If the same effort was applied to tidying up the forum (recipe sections) a bit, wouldn't that be better?
Marketing-wise, though, I agree a tidy pdf document e-book does have a certain appeal.
I also agree it could cause a few disputes, e.g. why one person's recipe is included, and another left out. I don't see how it makes sense for a book to include more than one recipe for each dish. It may be difficult to decide which single recipe to include for naan bread, for example, when people have quite strong views as to the best approach. A forum or web-based recipe site can reflect that, and give lots of options, but a pdf document or published book can't, unless it starts to look untidy.