This was actually the first book I'd encountered in the lab with padded boards. The cover material felt like paper lined vinyl. The material was cracked in a few places and was coming off the padding. The spine had almost entirely deteriorated. Luckily, the sewing, original mull lining, and joints were in tact.
To recover the book, I used a sheet of Morike. I glued it down only at the spine and the turn ins, to prevent damage to the padding. Finally, I glued the original covers on the Morike (PVA). I did not bother to tone the Morike because I was afraid it would do more harm than good.