Super Glue!!! No I'm just kidding. I would probable go to your librairy and ask them if they know any good book repair-men. Most of the time librairys are in touch with poeple who can fix all kinds of books, Even manga!
Buy a new one, it's probably cheaper! And use the old one for whatever you want (if it's really totally falling appart, I mean)! The pages that are falling out could be used as pin-ups, or perhaps even decorative covers for school/work folders? I used to do stuff like that with really trashed copies of comics, when I was in school. I have a few manga with loose pages too (mostly pocket volumes of Sailor Moon manga that were not bound well), but I feel like I can live with it so long as I look after them carefully and don't loose the pages...