Chapter 3 provides a very nice introduction to the semantic elements of HTML5 by evolving a HTML4 document, also a syntactically valid HTML5 document, into a more desirable HTML5 document, making use of new semantic elements.
Chapter 5 is a rather thorough explanation of the semantics of the <audio> and <video> elements. There is ample detail about containers and tracks — of audio and video packaging — that it completely demystifies the handling of audio and video in HTML5.
"Everybody knows about web forms, right? ... You don't know the half of it ... HTML5 defines a dozen new input types ... I don't mean to say that all of these exciting features are actually supported in every browser ... In modern browsers, yes ... In legacy browsers ... all of these features degrade gracefully in every browser. Even IE 6."The italics are mine, for emphasis.
The online version of the book is made available free under a Creative Commons License.
There are many other reviews of the book on the web, and I found this one by Ben Nadel particularly enjoyable to read.
This is definitely a book for your bookshelf, physical or virtual.