This is the best book out there on XML. If you want an intro to XML get Mark Johnson's article "XML for the Absolute Beginner." If you want to go in depth, this is the place to go.
Unlike the hundreds of other books that just describe what XML and XML Schema are, Daum's book gets deeply into why it is the way it is, by exploring how it fits in with conceptual modeling and how you would fit it into a complex environment.
He introduces a beguiling simple example domain (jazz musicians) which he revisits over and over again throughout the book. Through this simple example he works out subtle differences in different approaches to modeling, to schema construction, to constaint definition and modeling and finally how would you map this back to Object or Relational technology.
He introduces Asset Oriented Modeling, which is a form of conceptual modeling more attuned to XML schema model creation. His treatement of polymorphism in Schema, and techniques for evolving schema are as good as I've seen anywhere.
Excellent treatment of constraints, both conceptually as well as practical approaches to realizing the constraints in XPath or XSLT. These were worth the price of the book by themselves.
It's probably too late, but if you get no other book on XML and XML Schemas, this is the one to get.
