I would suggest the following books, select depending on your level of XML experience.
Beginner : Learn XML in 24 hours Sams Series - Michael Morrison.
Medium: Beginning XML
High: XML Quick Reference by Skonnard, Prof. XML 2nd ed.
Read various links available at the top of the XML Certification main page. That should give you a better idea. Still you need any info, feel free to ask.
All the best !
Rajan

:roll:

[ January 26, 2003: Message edited by: Rajan Thiyagarajan ]