I am new to the book preview scene, but, I was excited for my first preview. I finished working my way through HTML5 for .Net Developers (http://www.manning.com/jackson/) preview for Manning Books. This book was written by Jim Jackson II and Ian Gilman and is a very good source for .Net developers like me who do not have a lot of experience with developing for the web. I was very impressed with how in depth the book was, especially with all of the roadblocks that HTML5 currently has with web browser support.
Pro Tip: Use multiple browsers to work your way through the code (I know, obvious to web developers). There are good resources on the web that tell you which browsers support what, but, I learned early on that I should test the code against more than just IE.
One aspect of the book I really appreciated were the samples. Each sample is annotated to help you understand by explaining the samples. I found this really useful when trying to follow along. In reading this book, you will be shown many of the differences between HTML4 and HTML5, complete with code snippets, diagrams, and illustrations.