The W3C XML Schema Specification primer
http://www.w3.org/TR/xmlschema-0/
The W3C XML Schema Specification for structures
http://www.w3.org/TR/xmlschema-1/
The W3C XML Schema Specification primer for datatypes
http://www.w3.org/TR/xmlschema-2/
Microsoft's MSDN XDR Schema Developer's Guide
The place to start for XDR schema development
http://msdn.microsoft.com/library/psdk/xmlsdk/xmlp7k6d.htm
A discussion of XDR Schema
Dated from 1998 but still a good resource
http://www.ltg.ed.ac.uk/~ht/XMLData-Reduced.htm
