"Obviously the vast majority of my OB content can't go into the book, because there's so much of it."

I know this is not what you asked for, but I'd like to vote for a long book. I feel that the kind of people who will be interested by it (and readers of OB) probably won't be intimidated by the page count, and I know that I'd really like to have a polished paper copy of most of the OB material for future reference. The web just isn't quite the same.

In short: Something that is Godel, Escher, Bach-like in lenght probably wouldn't be a problem, though maybe there are other good reasons to keep it shorter other than "there is too much material".