These guys put together a wonderful book; I've find it an ideal guide to solving pitfalls using XQuery. Having this book on my desktop help me in preventing potential problems with my code and saving countless hours of time tracking down issues that aren't initially apparent.
I don't think I've read a computer book as poorly organized as this one. The basic premise seems to be present some random aspects of XQuery or related topics in a chapter and have no flow through the book. There is a lot of good material in the book but try to find it - it's more or less impossible because the index is abysmal. There's no real introduction to how to use XQuery - instead we start off with XSLT. That may be OK (they are related) but it seemed a bit strange to me. Another problem with the book is that some of it is out of date. The XQuery specification is a work in progress but the book was written after (and refers to) the final draft - but a lot of the examples are not compliant with that final draft. The best thing that I can say about this book is that it forced me to research issues more deeply and therefore find out what I needed to know. I wouldn't buy this book - try to borrow it and see if it meets your needs.