A knowledge of XML is essential if you want to build applications around the Office XML capabilities rather than just using other people's templates. If you're already acquainted with XML, you don't need to read this appendix. If you're not, you should read on.
The general overview of XML given in this appendix should be sufficient to enable you to work with XML documents. For a much more solid grounding in the many details of XML, you should consider these books:
You may also be interested in the "Annotated XML Specification," written by Tim Bray and published online at http://xml.com/, which an provides illuminating explanation of the XML 1.0 specification. You may also look to "What is XML?" by Norm Walsh, also published on XML.com.