Here's another book that was just recommended to me:
Querying XMLXQuery, XPath, and SQL/XML in context (The Morgan Kaufmann Series in Data Management Systems)by Jim Melton, Stephen Buxton
Foreword by Don Chamberlin
Although the book was written BEFORE the release of DB2 9, it is an excellent product-indepentant book that can help you learn how to query the XML you wish to store in a database.
From the Back CoverThere is no more authoritative pair of authors on Querying XML than Jim Melton and Stephen Buxton. Best of all, as readers of Jim's other books know, his informal writing style will teach you what you need to know about this complex subject without giving you a headache. If you need a comprehensive and accessible overview of Querying XML, this is the book you have been waiting for.-from the foreword by Don Chamberlin, IBM Fellow, Almaden Research Center