The following postings are discussing aspects of XQuery and its Type System:

1. Discussion of the problems of the syntax and semantics of the XQuery SequenceType in the Nov 2003 XQuery Last Call document.
2. Series on the XQuery and XPath 2.0 Type System
• An Introduction to the XQuery (and XPath 2.0) Type System: The general concepts
• An Introduction to the XQuery (and XPath 2.0) Type System: The XQuery types
• An Introduction to the XQuery (and XPath 2.0) Type System: The Impact on XQuery and XPath

More will follow which I will add to this post, so please check back frequently.