There was a time when all good books would be thread sewn and perfect bound. However, the cost of thread sewing led many publishers to opt for perfect binding only for many of their everyday publications, such as computer manuals. This glue-only process was fine in many ways but, if the book was handled roughly, pages could fall out.
PUR binding is a superior type of perfect binding. It uses a more flexible glue giving greater adhesion, a more robust book, and even the ability for the book to lie flat when open. This type of binding is becoming more and more widely available.
For the anoraks among us, PUR stands for Polyeurethane Reactive.