Great news! O'Reilly has been kind enough to let me distribute my eBook Unobtrusive Ajax to readers of my blog, so now you can download and read it for free!
From the book's cover:
Nowadays, I would also suggest bearing different user interfaces in mind. Not every visitor to your website has a mouse and a keyboard, some are using their handheld touchscreen phones, and if your shopping cart requires people to drag and drop, you may end up losing customers. Be careful not to make assumptions about your visitors. If you stick with basic HTML, you can't go wrong.