Oftentimes, I read something and start to ponder questions of the form "Does the formula on pg. 87 have a typo?" or "Isn't this a gap in the proof?" or "Why does the definition have this extra assumption?", etc. Learning questions which I would generally consider a bit too specific or trivial to post on MathOverflow but I wished did have an answer out there. And I suspect others reading the same thing may have also wondered about it.
If there was a subforum dedicated specifically to book X or paper Y, then I wouldn't hesitate to ask. It would be nice for people studying the same thing to have a central place to exchange ideas about it. And MathOverflow seems like an ideal venue for this kind of thing.
Would such an idea be feasible with the current engine? Allowing people to create subforums in the site dedicated to fielding specific books and papers, that is.