XML in a Nutshell - includes XSLT & XPath, XLINKS, XPointers, CSS & XSL
Mastering XMI - I found this book a bit heavy going, it seems to spend a lot of time talking about the importance of standards, introducing XML, UML and OMG. However I found it hard to pick up the essence of XMI, why is it important and how does it work. The book concentrates a lot on IBM programs such as WebSphere, XML4J and XMI framework (not much on JMI) as you would expect from a book written by IBMers.