An Introduction to XML and Web Technologies
Anders Møller
Michael I. Schwartzbach
A must buy!
Object-Oriented Type Systems
Jens Palsberg
Michael I. Schwartzbach
The best (and possibly only) book I've seen on the topic. I very
much liked their approach of starting with a simplified language
and adding the necessary features. The algorithms are useful,
well presented, and their assumptions are laid out clearly.