This is an overall good introductory book to HTML5 for existing web developers. If you are completely new to HTML and want to start with HTML5, then this is not the one you should read. This is not a step-by-step book, but it introduces the background, the rationales, and the new features of HTML5 with appropriate amount of depth to get started.
The book only has around 200 pages, it does not go very deep into any topic it introduces, but the amount of information it gives is enough as a starting point. At the end of each chapter, it also gives a pointer for further readings. A lot of contents are of very high value, not only teaching how to do, but also telling the reasons. And it's very thoughtful to include the matrix for browser support for the features it introduces.
In general, for any existing front-end developer, learning and accepting the concepts of HTML5 shouldn't really be particularly hard with the right information, and this is a book I recommend.
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