At least this is the first review of the Professional XML book that I have seen - there may be others. But Tech-Unity did a review and got some of the purposes of the book right on the button -
"This isn't a book about learning XML. This is a book about integrating XML into advanced programming and development. As the blurb on the back goes, "Wrox Professional Guides are planned and written by working programmers to meet the real-world needs of programmers, developers, and IT professionals". If that describes you and you need to add XML (and who doesn't these days) to your skill set and your projects, you're gonna love this book."
We made a big effort in making a book for the programmer that wants to make the best use of XML in their applications TODAY. This book is full of a lot of examples for both the .NET and Java developer in helping you to make that happen.