I used to work at Spyglass, where the Mosaic web browser was invented (and ultimately sold to Microsoft). A few points: 1) HTML is, indeed, a specific instance of XML. 2) You CAN have multiple authors for a book just as described. 3) XML is more of a heirarchical database structure because the data is held in files rather than columnar tables and you must "traverse" the files via nodes to access the data, 4) XML can be slurped into a database (SQL Server, for instance) for faster performance. But by itself, XML sitting in files is not a fast way to access and manipulate data.

scott