Where There's Free There's Me!: Dive Into HTML5

I came across a free book on HTML5 this week.  I started reading it, but it's pretty long so I'm going to finish it up sometime in the future.  From reviews and reading the first chapter I can already say it's going to be good.  Mark Pilgrim wrote Dive Into HTML5.  It is released under the CC-BY-3.0 license so it's free to read online.  If you love holding a paper copy you can also buy the print copy from Amazon.