Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.
Schematron is a simple tool that anyone working with XML should have in his toolbox. Its basic structures are basic, and its flexibility is immense.
If you are a W3C XML Schema user, you will find Schematron most useful in working around W3C XML Schema limitations, including issues such as checking for a specific root element, forbidding xsi:type attributes, dealing with co-occurrence constraints and handling unordered content models.