If you collect books, leather books are something special. It is very difficult to keep (old) leather book bindings in good condition. The National Library of The Netherlands has a very interesting article on its website. It is called “Guidelines for the conservation of leather and parchment book bindings”.
It is a bit difficult to read due to the lack of white space, but it seems very interesting. I have not read all of it. It is a bit dry here and there since it is written from a scientific point of view. You can find the lengthy article here.