Jeremy Keith's HTML5 for Web Designers is up for free web perusal.

The book is quick and clear -- a high-level overview, much punchier and more readable than the full spec.

I'm a bit of a web fogey, and found Keith's book spot-on what I needed to catch up.

Paul Stamatiou reviews the printed version here, but the whole book is so short (85 pages) that you'll waste barely any time paging through its 6 brief chapters.