If you're familiar with older versions of HTML, you can head directly over to the W3C site to learn the changes for HTML5. HTML5 is currently still in development.
The first online book I had read about HTML5 was Dive Into HTML5 by Mark Pilgrim. If you know a lot of the details of HTML4 and XHTML, you could learn a lot from it. I don't recommend it for HTML newbies.
Next, I'd suggest this site by Google: HTML5ROCKS.
- Save $50 on Paint Shop Photo Pro X3 Only $49.99. Sale ends soon. Make great web graphics.
- First Things to Read when Learning HTML5
- Getting Started with HTML5
- HTML5 News
- HTML5 Resources
- HTML5 Tutorials
- HTML5 Demos and Galleries
- Learn to be Web Designer without Going to College - It's a free course!