My involvement with XML and SGML:

Good Introductions to XML

The SGML/XML Web Page is the ultimate reference on all that's available in the world of XML and SGML. (Note that it's searchable.)

Good Introductions to SGML

Start with the SGML FAQ, which is very short. The Text Encoding Initiative's Gentle Introduction to SGML (a 77K text file--also available in Russian!) is also very good.

Not the SGML FAQ

"The elements be kind to thee, and make thy spirits all of comfort!"
Anthony and Cleopatra, III ii