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If you want to read more about using the DOM with C++ and MSXML, see Chapters 2, 3, and 5 of Using XML with Legacy Business Applications (Addison Wesley, 2003, ISBN 0321154940). The book also has tutorials for XSLT and reading XML schema language, and covers several issues related to building XML support into legacy business applications. (Preview this book on Safari)
The World Wide Web Consortium's DOM Level 2 Core Recommendation, as well as being the authoritative source, is also an excellent introduction to the DOM's main interfaces and methods. The MSXML 4.0 SDK is also an excellent reference for the MSXML implementation of the DOM.