Quite by accident, I discovered that Eve Maler and Jeanne El Andaloussi’s Developing SGML DTDs: From Text To Model To Markup is available online. I’m one of the people lucky enough to own a hard copy, but if you aren’t as fortunate, read it at http://www.xmlgrrl.com/publications/DSDTD/. It’s one of the best books ever written about information analysis, that (far too) little used skill required to write a good DTD. In my ever-so humble opinion, the book should be mandatory for anyone involved in a markup-related project of any kind, that’s how good it is.
(Yes, I know it was written before XML came out, 12 years ago, but XML is SGML, really, and the book remains as useful today as it was when it came out in 1995.S