Regarding SGML, XML and the DocBook-DTD there is lots of literature available both on the internet and as printed media.
The following URLs provide information that has been found useful for introductional purposes. As a matter of course the internet provides many more informational sources and a google search will reveal hundreds of links for the various topics.
http://www.oasis-open.org/docbook
The DocBook home page.
http://www.oreilly.com/homepages/jasper/DocBook/over.htm
An overview of DocBook-DTD.
http://www.oasis-open.org/docbook/documentation/reference/html/docbook.html
The definitive guide to DocBook written by Norman Walsh.
Very elaborated material regarding XML.
http://www.tm.informatik.uni-frankfurt.de/PSWWW_2002/Paper/JudithWinter-XML.pdf
A good intoduction to XML (german).
Home page of the world wide web consortium.