O'Reilly has just released a book on Schematron by Eric van der Vlist as part of their Short Cuts series of short, inexpensive PDF books. I'm sure this book will do well, because Eric knows his stuff and Schematron is remarkably useful.

When I was at LexisNexis, they were using XML all over the place, and I was always encouraging people there to use the latest advances in XML technology. Schematron was always the easiest sell. (I just heard recently that their use of it has increased since I left.) An ISO standard rule-based XML quality-checking diagnostic system with natural language error messages that only needs an XSLT engine to implement—what's not to like?

Its name gives the impression that it's an alternative schema language, and strictly speaking it might be, but I see it more as a complement to existing ones. I've even made a case that Schematron can add enough to a DTD-based system to let the system's users postpone a transition to RELAX NG or W3C Schemas as a replacement for DTDs. An, when implemented as an additional layer to an existing system, a Schematron rollout can mean very little disruption to that system.

For anyone interested in maintaining and improving the quality of their XML, you owe it to yourself to check out Schematron. Congratulations to Eric and to O'Reilly for getting the book out.