Don't be swayed by the negative review up there at the top - If you're a programmer you should probably check out something from O'Reilly, and you should read the name of the book before you buy it. This book is aimed squarely at *web designers* and is intended to be an overview of the subject with just enough technical and spec information to get you started.
I'm halfway through the book, and I highly recommend it for anyone on the web design side of the industry who's interested in XML but not really familiar with what it is or the fundamental roots of HTML.
The book goes over what XML is and what it isn't, how it compares to HTML, how it relates to CSS and XSL, potential applications for XML, DTD's, the W3C recommendation process, and lots of other related information.
I find these many different angles quite helpful, because my perspective is limited by comparison; HTML, JavaScript, and graphics occupy the bulk of my time, and I never would have thought of some of the considerations and advantages of XML brought up in this book.
If you're a web designer, buy this book. If you're a programmer, go look for the O'Reilly book!