Concur with Steve. Museums seek to preserve rather than fully restore. Old restorations have caused more damage and it's best to arrest any degeneration. Apply Rennaisance wax on both front, back and sides.
Hint: wash hands with hot soapy water to remove any oils or grease that may stain the leather. Then after laying the item to be preserved on a clean surface, apply the rennaisance wax. Some conservators wear white cotton gloves or even allergin free rubber gloves. Cotton is good enough for me but in the absence of it, wash your hands.
I certainly did before I handled any documents at the National Archives. I handled the documents only to remove them from their folder or flip them over to read. One thing I didn't do was hold them up to read or make an impression on them while lifting them.