It's an "okay" book, not ground-breaking, but a pretty good and concise overview. There isn't that much new stuff about HTML5, so most of the book is padded with general accessibility tips and a section about screenreaders, which I really liked. WAI-ARIA is given a chapter, but it's more of an invitation to do your own research than a thorough overview.
Summing up: good book in itself, but if you're an accessibility pro, don't expect to be awed.