This book answers two questions. The first question is "Why should I make my web site more accessible?" If you do not have a web site, this book is not for you. The second question is "How can I make my web site more accessible?" If you are not convinced by the first answer, you will not be interested in the second.
The above book, from my perspective, is required reading for anyone interested in Accessibility. It's not that long and it is the most enlightening book out there in regards to Accessibility. Read it today!
It's free and, it's free to distribute.
This book is free. Permission is granted to copy, distribute, and/or modify this book under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the last chapter of the book, entitled "GNU Free Documentation License".