It’s a simple idea, really: accessibility + usability = usable accessibility.
Once we remove the barriers to access addressed in standards and regulation, the next challenge is to make communications, technology and other products inclusive and usable by everyone.
A good place to start reading about usable accessibility are these resources:
Just Ask: Integrating Accessibility Throughout Design by Shawn Lawton Henry covers accessibility through the user-centered design cycle, including personas and usability testing with people with disabilitites. (Online and print versions)
Designing for Inclusion at the W3C Web Accessibility Initiative has good examples of how people with disabilities use the web and other technology, along with research collections on older adults and mobile users