Ok, it's not a book exclusively about web accessibility. But, in this book, David Dylan Thomas shows us how dangerous our bias can be and guides us on how to make better invisible choices.
Sheri Byrne-Haber wrote this free practical handbook for designers about
(PDF and audio versions)
This is a comprehensive guide for those who need practical tips for designing
digital products more accessible.
Personal note: even if that guide is about starting accessible digital products, I profoundly regret it was designed (and validated!) with so poor contrast-ratios!
Free e-book (pdf format)