SGML and XML as Meta- Markup Languages
SGML and XML are formal metalanguage facilities for defining markup languages. SGML has the full power to configure a set of features for markup languages, whereas XML has a fixed set of these SGML features. Since XML is a profile of SGML (rather like a proper subset of SGML features), most introductory presentations about SGML convey relevant information about XML as well.
A collection of references for SGML may be found in "SGML: Introductions and Overviews."