The other day I was at the bookstore (my wife had organized a book fair for my daughter's school) and decided to check out the books about HTML5. I had read a few things in the past but wanted a deeper understanding of what changes it brings to writing web applications. I grabbed the five books I could find on the subject and started browsing through them. Introducing HTML5 (2nd Edition) hooked me from the introduction and I ended up buying it.
The introduction provides a history of the development of HTML5. I had thought of HTML5 as the evolutionary successor to HTML4 but was interested to find that not for a small group at Opera lead by Ian Hickson it almost wan't a reality. W3C was pushing XHTML 2.0 as the successor to HTML 4 which required XML syntax and broke backward compatibility. Whereas the Opera group took an approach more focused on current browser behavior and looked to add items to simplify web development.
The first chapters deal with the changes to overall structure of an HTML document and text on the page. There are chapters on the new form elements, video, the new canvas tag and offline storage. There are a couple of chapters on geolocation, messaging, and sockets that I haven't gotten to yet. Finally there is a chapter on backward compatibility and using polyfills to make sure your pages work with earlier browsers.
Overall it's a great overview of the new stuff in HTML5, why the new tags were added and when to use them.