Mark Pilgrim has taken his multi-part series of articles on making websites more accessible and made it into a book form. He says:
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.