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Bless Joe Clark.
Not only does he write a stunningly useful book, but he also provides all of the chapters (for free) to Building Accessible Websites.
I was led this way since in the section on accessibility in Zeldman's Web Standards book, the mighty Z referenced Joe's work with supreme praise.
I've just dived in a little, but Joe writes extremely clearly with a little bite of humor. And it is practical stuff you can use. I am already reviewing my coding of web forms based on Chapter 12.
If you think web accessibility is just tossing in some ALT tags or getting Bobby approval, you better dive in a little deeper. It is not hard at all. Like someone has mixed the metaphors, "It is not rocket surgery." Joe leads the way.