The stories inside books can last forever, but the books themselves can deteriorate. Antique leather book covers can crack and dry out. Many families are concerned whether if their family heirloom Bibles and rare books can be restored. Read on and see if you can save your precious book.
Perform "triage" on the leather book you want to restore. Look at the wear to the leather. If the cover is completely rotten, with crumbling leather and little shine left, there's not much that can be done to restore the cover. You may want to consider having the book rebound. This may be best left to a professional or someone with experience in bookbinding. If the damage is not so severe you may be able to restore the cover on your own.
Consider using a cellulose based consolidant. This product (hydroxypropyl cellulose) will help bind and improve the appearance of drying leather. It will also keep it from deteriorating further. Consult instructions and material safety data sheets prior to using this product.
Mix hydroxypropyl cellulose in a solution with isopropyl alcohol or a similar solvent. Use a proportion of 1 1/2 tsp. powder for every cup of solvent. Apply to leather book cover evenly with a soft brush and allow to dry.
Use a polyester film jacket to protect the book further. Obtain a polyester sheet larger than the book you want to protect. Trim so that the cover is the same height as the book in the middle of the cover and approximately 1 1/2 times the width of the book near the edges (the cover will resemble a sideways letter "I"). Crease and fold the flaps around the cover so that the leather book is protected from wear.
Note that consolidants are primarily meant to prevent further decay of a book. They may not restore a book to its original shine.
Use leather dressings at your own risk. These may improve the appearance of a slightly worn leather book, but evidence suggests that they may dry out the leather in the long run.