A mountain of knowledge,
This review is from: Building Accessible Websites (Paperback)Depending on your approach to website design and development, accessibility is becoming a very 'hot-topic'.
One very good book is Steve Kruggs "Don't Make Me Think"; another is this offering by Joe Clark.
There isn't anything Joe leaves unturned while taking the reader through the myriad of web-based accessibility. Not just referring to the obviousness of visually impaired visitors but also those with learning and motor difficulties for whom reading, navigating and otherwise doing what most of us purely take for granted.
This is a very detailed book and shouldn't be considered a light-read but as a valuable resource for any website builder who plans to run ecommerce, charity or public information websites.
You don't HAVE to read from cover-to-cover but this book should be on your shelf as it's worth checking out at least once in your life.