Elmer's glue. Dab it in the binding with a toothpick or use small drops.
I once bought a textbook on Mathematical Physics that had about 12 leaves missing (~24 pages). Fortunately, I found another edition with the pages and just Xeroxed them.
For any really valuable book (collector's item or heirloom), then you definitely should have a professional do it. If you just don't want the thing falling apart, the aforementioned techniques (except duct tape) will work fine.