|The Debian SGML/XML HOWTO|
SGML in general
XML in general
FreeBSD Documentation Project Primer is a nice introduction to SGML and DocBook
Simplified DocBook, a version of DocBook with less elements to learn
Different customizations or extensions to DocBook
Other operating systems: this section will list documents similar to this HOWTO (I mean practical documents) for operating systems other than Debian.
RedHat users of DocBook should probably see the Cygnus tools.
For FreeBSD, if you want to use DocBook, see their FreeBSD Documentation Project Primer for New Contributors, specially its list of mandatory tools.
SuSE's has some tools to process DocBook documents easily.
It is not operating system-specific but O'Reilly has a nice documentation about their publishing system.
 SGML CD, Bob DuCHARME, Edited by Prentice-Hall, 0-13-475740-8.
 DocBook: The Definitive Guide, Norman Walsh, Leonard Muellner, Edited by O'Reilly, 1-56592-580-7.
I didn't read it yet. The entire book is also online.
Why DocBook doesn't have a humour element?