There are now over a dozen Ajax books out there. Were you able to read any before or during writing yours? Did you make a conscious effort to do anything better, or differently, than any of the others that existed at the time?
I haven't gone through the book, dont know when I will. But I would place this book as a first read for beginning and mastering Ajax. Although Ajax in Action is another good one for beginning and mastering Ajax, as Linda said Head Rush is in the "Head First" format, that makes this book stand out.