There's nothing in here that you can't learn by printing off a stack of the W3C site and reading through that. On the other paw, this book is small, convenient, and forms a valuable handy reference for most occasions.
It's useless as a teaching guide - but then no-one buys O'Reilly nutshells for that.
It's useless as a learned tome of ultimate authority. That's what W3C TR's are for, and this won't replace them.
It's not enough to cover XSLT to any useful depth, although the XPath section goes much of the way. Of the many small and fragmentary aspects of XML; XSLT is the one that benefits most from having a large stack of examples, rather than a reference.
XML Schema is ignored. It's churlish to complain too much, because it's a fast moving field and Schema was one of the fastest moving topics in it. Still, it's definitely the weakest point of the book. If you're working with schema development, then you'll need another reference for that, and it's a topic where reference books get a lot of wear very quickly.
On the whole, any XML beginners will find this a valuable book to have, provided they have a few others too to learn from initially and they're prepared to discard it in 6 months, once they've outgrown it.