I would agree that this is a handy book. The main question is how it rates up against O'Reilly's XML in a Nutshell who's style this book seems to emulate. Personally I think that this is a much better book, being more up to date. The UML diagrams for the DOM and SAX interfaces are a nice touch.
One thing XML in a Nutshell covers that this book doesn't seem to is XSL FO. I'd also like to see XML in a Nutshell's second edition updated to include a section on SVG.
Personally I use XML in a Nutshell all the time, but then I'm biased as I did the tech review for it. Mostly I just use it to look up characters in the entity reference at the back ;-)