Interesting question, and it's one I'd like to answer a little more fully than you might expect.
A lot of the books -- and material online -- about Ajax assumes that you really fundamentally understand what Ajax is about. I'm not just talking about knowing how to code JavaScript or JSON, but about really getting the Ajax programming paradigm. Unfortunately, I think most people -don't- really get that paradigm.
Ask most people what a web browser is, and they'll tell you it's the program you use for viewing web pages. But it's far more -- it's an entire execution environment for displaying, rendering, and running several different languages (HTML, CSS, JavaScript). And until you really get a hold of the browser, and its role in web applications, most of what you can learn about patterns and usage of Ajax is really sort of useless; you'll use the right pattern without understanding why.
So the long-winded answer is that in some ways, Head Rush Ajax is an introduction to Ajax. But, it's the introduction that most programmers never got. So we spend a ton of time on asynchrony, because -- and this is the truth -- most programmers know what asynchrony means, but don't really understand the issues involved. Google Maps, for example, uses Ajax, but isn't at all asynchronous in any real sense. Ditto for apps like Flickr that we've long held up as Ajax poster-childs. They're great apps, killer, interactive, very Web 2.0, but not asynchronous. Read Chapter 3 of Head Rush Ajax, and you'll thoroughly understand why -- and how you could change them to be asynchronous.
So that's the philosophy of Head Rush Ajax. We want you to get these "basic, intro" concepts (terms that I hate in this context) in a way that means you really understand them. And along the way, you'll obviously pick up lots of patterns and watch-out's. There's a chapter that deals extensively with using two request objects, managing multiple requests in JavaScript, and multiple asynchronous calls at one time. But we don't call it a pattern, because it's just a natural thing you have to know in asynchronous programming.
We spend tons of time in Chapter 6 talking about when XML makes sense, and especially when it doesn't; that's a pattern, but it's presented in an unusualy way (the server sitting on a couch with his shrink, believe it or not!).
So I think you'd be pleased and challenged. A cursory review might make the book seem basic, but a thorough reading will leave you with a strong understanding of Ajax, and you often won't need patterns from other books -- they'll be obvious to you because you understand the core Ajax paradigm so well. Then, if you pick up some other books on Ajax (which you should!), you'll see beyond the surface details. And that's a good thing :-)
Thanks
Brett