As parts of the HTML5 specification are still being hammered into their final form, books on the subject have limited utility. Broadly speaking, your best guides will be technically informed, up-to-date sites such as WHATWG and HTML5 Doctor. However, most of the spec has been completed, so with some limited reservations I can recommend the following books.
(A note that both books assume that the reader has some previous experience in creating web pages. To the best of my knowledge the canonical “from scratch” HTML5 manual has yet to be written. If you think you’ve found one, please let me know in the comments section below.)
Again, O’Reilly comes to the fore with HTML5: Up and Running. What may not be obvious is that the core text of the book is derived from one of the first, and best, HTML5 evangelist sites, Dive Into HTML5; the author of the site, Mark Pilgrim, largely took the original site text and edited it into dead-tree format.