From a few years back when I was checking into this for my folks:
Disabled Travelling
Society for Accessible Travel & Hospitality
www.sath.org
lists destination-specific, handicapped-friendly guidebooks, travel agents and tour operatiors
www.access-able.com, search by country to find handicapped-accessible hotels, modes of transport, and local attractions . Also links to newsletters, mags, and orgs.
Global Access Disabled Travel Network
www.geocities.com/Paris/1502
first person accounts