Interesting question, and it's one I'd like to answer a little more fully than you might expect.
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.
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 :-)