HTML5: Up and Running - A fun-to-read intro to HTML5
This is a well written, fun to read book on what is new in HTML 5, why it's important and how it came into existence. It introduces new tags, reminded me of some old and forgotten and, in general, improved the quality of the HTML I write. It also presents Modernizr, a feature-detection framework (which is a much better idea than trying to parse the user-agent string). The book has an associated website where most of its text is available (albeit in a less couch-friendly format than the book/e-book and that does little to support the author) and both book and site have been important companions on my quest to relearn HTML and to get rid of any bad habits I may have developed in the past 15 years.
You can find the book at O'Reilly's site.