I've been very impressed with Aaron Skonnard and Martin Gudgin's Essential XML Quick Reference: A Programmer's Reference to XML, XPath, XSLT, XML Schema, SOAP, and More. It's in the same style as an O'Reilly nutshell book. It's very up to date with some handy UML diagrams. The examples are in Java and Visual Basic which is interesting as you can see the same interfaces being implemented in two entirely different languages.
Still waiting for a hardcore Perl/XML book. :-( Perl's powerful way of handling data structures make it feel to me like the perfect language for working on XML, but Java seems to be doing a very good job of selling itself as THE language for XML.