If you've been writing HTML for a while and are interested in learning Extensible Markup Language, or XML, then this book is aimed at you.
XML: A Primer is divided into four parts: Getting Started, Describing Documents, Supporting Specifications, and Building Your Own Markup. The initial part delivers, in a fragmented way, a short history of HTML, XML and their roots in Standard Generalised Markup Language (SGML). In the remaining chapters you are given an explanation of how XML is used as a markup language, including a discussion of its elements and attributes.
Part II is where you start learning about document processing. At the core of document management is Document Type Definition (DTD), which is handled in a clear and thorough manner.
One note about the book is that since it is a handbook size, much of the sample code runs over single lines and pages, making it more difficult to read.
The section on supporting specifications is concise in its approach to identifying how XML can be processed, linked and queried. Included here are a number of tables, diagrams and screenshots, which do a good job of explaining and demonstrating concepts; however, they, too, are hindered by the book's size and often appear too small.
In the final part you are given a tutorial on how to build your own markup, including how to reformat Web pages as XML documents. The chapters focused on applying XML for commerce, document management and data-driven applications are particularly interesting. This is where you see some real-life examples of how XML can be used like other scripting languages to perform business tasks.
Overall, XML: A Primer, is probably best suited to someone with at least some experience with XML. However, its brief discussions and small size make it just the ticket for a reference publication.