SitePoint's newest book is No Nonsense XML Web Development With PHP by Thomas Myer.

No-Nonsense XML Web Development With PHP dispenses with the boring, academic-style writing about XML theory and presents real, practical uses of XML that you can apply to your existing Websites today. The book will teach you, step-by-step, exactly how to:

Create a full-blown Content Management System (CMS) based on XML.
Use XML-based data formats to manage the storage and management of all of your content. You'll see where the versatility of XML can make storage, retrieval and formatting of data much easier.

Create a dynamic site map using XSLT.
XSLT allows you to transform your existing XML-based data into new formats and modify its structre. You'll learn how to generate a dynamically-updating site map from your existing XML data.

Generate XML feeds (including RSS) for your Website.
Generate and serve error-free RSS feeds the easy way using PHP's XML generation capabilities.

Set up XML-RPC Web services on your site.
Facilitate transfer of important information between disparate systems. XML-RPC (XML Remote Procedure Call) allows you to securely execute queries and functions on remote servers.

The book teaches you exactly why XML is so powerful and versatile, and you'll see just what PHP 5.0 is capable of achieving with XML. The book quite simply teaches you what you need to know about XML in order to use it effectively

You'll learn how to:

Write portable XML documents that can be re-used across applications.
Use XML namespaces to combine data formats in many ways.
Use DTDs to strictly define the structure of your XML documents.
Use XSLT to transform your existing data into other formats.
Write XPath queries to retrieve data from an XML document.
Use XML in conjunction with databases.
Use the Document Object Model (DOM) to manipulate documents.
Rapidly process XML using PHP 5.0's built-in SimpleXML functionality.
Use fast SAX parsers to read an XML document incrementally.