I own a number of books published by Origami House. And of all those books, it's only the Origami Insects II that's falling apart. Bad batch perhaps.

In my case, the latter 1/3 set of pages fell off. I squared them off and held them between two plywood sheets. Then I roughened up the edge that comprise the spine, and smeared it with a generous amount of elmer's glue. After drying, i glued the whole thing back to the book. I was able to get a good fit and the repair was hardly noticeable.
Luckily, the original 2/3 is holding up well.