HTML5: Up and Running
Mark Pilgrim has written a really useful introduction to the new technologies that are driving HTML5 adoption. If you're creating HTML for web sites then I recommend this book.
I'm not a web site designer, but I've always tried to produce web sites that use valid HTML. And then when the current standard switched from HTML to XHTML I converted most of my web sites to the new standard. A couple of years ago, some friends started talking about HTML5 being a replacement for XHTML. I was tired of converting my web sites, so I pretty much ignored them.
And then last year more and more people started talking about HTML5. It was obviously time to investigate further. And Mark Pilgrim's book seemed as good a place as any to start.
It turns out that HTML5 is rather more than just a mark-up language. It's a term that encompasses a number of new technology standards that will be driving web application development for many years to come.
The most interesting new features to me are the native support for audio and video (although there is still some disagreement between browser makers as to which formats are supported) and the canvas element which will finally allow some powerful graphical effects to be produced in a manner which will work well across most (if not all) browsers.
I haven't yet had a chance to really start using HTML5 on my web sites. But when I do, this is a book that I will return to frequently.