While this is in no way a complete list of useful books on the topic of gardening with disabilities, I thought it might be helpful as a starting place.
Accessible Gardening For People With Disabilities, A Guide To Methods, Tools and plants
by Janeen R. Adil, Woodbine House
The Able Gardener: Overcoming Barriers of Age & Physical Limitations by Kathleen Yeomans, R.N., Storey Communications, Inc.
Container Gardening For The Handicapped, by Frank J Schweler, Hand-D-Cap Publishing
Designing Barrier Free Areas by Debbie Krause, Cornel University
Gardening for The Handicapped by Betty Massingham, Shire Publications
Adaptive Garden Equipment: A Resource Manual for Ptients, Families, and Professionals, by Craig Hospital Horticultural Therapy Program.
Further Information is also available through;
American Horticultural Therapy Association
362 A Christopher Avenue
Gaithersburg, MD 20879