This book is very useful and well written. The topics covered include XML basics, DTD's, style sheets, XML schemas, XML Query, XHTML, SYMM and MIL. It describes each topic in detail and with examples. Some of the examples help understand the different error messages given by the validating parsers. Perhaps that is the part programmers are mostly interested in.
Topics like XML Query and RDF are out of date as of now (Sept 2002), due to new W3C specifications, but introduction provided in the book is still useful for getting started. I really like XML Query chapter because no other small introduction is available elsewhere as of now.
This book is an excellent choice to get started with XML. It has got all the basics and nice examples.