I've written regularly about HTML5 for the last few years, but there's stuff in Ian's book I didn't know. It's meticulous and logical and builds a good momentum taking the reader along a well-pitched learning curve.
Ian's well-aware that, although he's showing some state-of-the-art stuff, not every user of your sites will be using the latest greatest browsers, so constantly reminds you - and shows you how - to be a responsible developer and serve your customers, not your ego.
JavaScript is my weakest point, so I found the scripts particularly well-explained.
If you're doing some work with HTML5 audio or video, you'll be wise to get this book.