One source of damage most people don’t think about when browsing for books in the library is the way they extricate the book from the shelf. When a book is repeatedly pulled by the headcap, the weak upper portion of the spine (called headcap) will eventually fail leading to tears in the spine and possibly even spine removal. Once the spine detaches from the rest of the book, damage to the book noticeably accelerates.
If the headcap is merely weakened but not torn, it is usually ok to send back to the stacks "as is". But if the headcap is broken or torn, it needs to come to the Preservation Unit for a repair.