SGML stands for Standard Generalized Mark-Up Language.
SGML is a standard for defining mark-up languages.
HTML is an instance of SGML.
SGML for Dummies - By Bill Von Hagen.
SGML: The Billion Dollar Secret - Companies produce large amounts of vital information in the form of documents. They are the life blood of the enterprise. Yet at most companies documents have not benefited from the huge investments made in computer systems the way other types of data have. This book is a collection of six case studies of visionary companies that have transformed their documents into assets through the use of SGML.
The Concise SGML Companion - Clear, comprehensive, and precise.