A 12 volume set of Gibbon’s Decline and Fall of the Roman Empire from 1823. Not only were the bindings destroyed by damp, volumes VI and VII had rotted and all the pages needed to be strengthened with Japanese tissue before they could even be restitched into books. The set had a great deal of family history and so it was definitely worth saving.
Many of the pages, particularly in volumes 6 and 7, needed restoration work even before they could be restitched.
