For info on SGML, you can ftp to either of two main archives:
sgml1.ex.ac.uk
ftp.ifi.uio.no
and look for the FAQ (Frequently Asked Questions file). An excellent
introduction to SGML can be found at these sites in the TEI (Text
Encoding Initiative) subdirectories (probably in tei/p3/doc). Look for
a file called p3sg.doc, which is "A Gentle Introduction to SGML".
If you want to pursue the subject more deeply you will find a very
useful bibliography by Robin Cover at the same sites.
Walter Henry
Conservation Lab
Stanford University Libraries