From Russel H Beatie's book "Saddles":
For those interested in the restoration of old, hard, brittle leather to a soft, pliable condition, I recommend John W. Waterer's "A Guide to the Conservation and Restoration of Objects Made Wholly or in Part of Leather", and Per E. Guldbeck's "The Care of Historical Collections", which has a section on leather. British Museum leather dressing can be obtained from the Fisher Scientific Company, Pittsburgh, Pennsylvania. Before using the dressing, it is necessary to clean the leather with Toluene (use care: Toluene is highly inflammable). Another leather restorer is Neutralfat SSS Leather Restorer, obtainable from the Ventron Corporation, Danvers, Massachussetts. Instructions for the use of these products can be obtained from the suppliers.
The book is from 1981, so I have no idea if all of these products are still available. I googled British Museum leather dressing and it seems to still be around. It also turned up this page: http://nautarch.tamu.edu/CRL/conservationmanual/File7.htm about leather cleaning and conservation. Good luck!