Most likely the covers have severe oil contamination.
Begin with clean Kitty Litter in a pan and leave the book sit on thekitty litter in a warm place for a couple weeks. Clay kitty Litter drinks oil out of things.
After that try a little corn meal with just enough water to make it into a stiff paste. Rub that paste onto pigskin, let it dry and brush off.
If the cover is leather (more likely) saddle soap is best.