SUMMARY: SGML formatters, non-document SGML use, bibliography
Norm Walsh (Mr. XML / SGML / TeX / O'Reilly/ GROFF / DocBook )
Amazon present SGML: An Author's Guide to the Standard Generalized Markup Language; Martin Bryan; Paperback
I have this book, and use it as my main reference to deal with any "tough" issues that come along.
It was one of the first good SGML references.
Amazon present The Sgml Handbook ; Charles F. Goldfarb, Charles Goldfarb; Hardcover
Charles F. Goldfarb's SGML Source: InFrequently Asked Questions (InFAQs)