This article is from the Publications about dogs FAQ, by Cindy Tittle Moore with numerous contributions by others.
Goldston, Richard, and Johnny D HOskins (ed). _Geriatrics and
Gerontology of the Dog and Cat_. W.B.Saunders, Co., 1995.
Hampton, John K. Jr., PhD, and Suzanne Hampton, PhD. _Senior Years:
Understanding Your Dog's Aging Process_. Howell Book House,
Pinney, Chris. _Caring for Your Older Dog_. Barrons Educational
Wilcox, Bonnie, and Chris Walkcowicz. _Old Dogs, Old Friends: Enjoying
Your Older Dog_. Howell Book House, 1991.
Frequently and highly recommended.