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 ;-)