Brittle books (paper suffering from acid deterioration) can easily be damaged if they circulate. You can tell if a book is brittle by doing a small double-fold (back-and-forth, back-and-forth) on a corner of a page and seeing if it breaks or becomes detatched.
It is best to only send damaged brittle books for Preservation Unit review and treatment . A brittle book that is not circulating and otherwise intact may last for years if stored properly. Such things as frequent circulation, photocopying, and interlibrary lending can easily cause a brittle book to become damaged.
Most frequent signs of damage would be covers coming off and pages breaking or coming loose. Some pages will break when handled.
Some brittle books that have circulated are in boxes. If a book is in a box that is not a publisher's box, in most cases it is in a box because nothing else can be done to it due to its damaged brittle condition. If you receive a damaged box back that has circulated, send to the Preservation Unit so a new box can be made.