The XHTML 1.1 Recommendation contains general information, information on compatibility issues, document type definition information, definitions, terminology and much more relating to XHTML.
XHTML.org provides XHTML development news and links to other XHTML resources, which include books and articles.
The XHTML School provides XHTML quizzes and references. This page also contains links to XHTML syntax, validation and document type definitions.
This site provides an article about XHTML. Key sections of the article overview XHTML and discuss tags, attributes and anchors.
The Web Developers' Virtual Library provides an introduction to XHTML. This site also contains articles, examples and links to other technologies.