I was first introduced to this when I wanted to work with the Subversion book source. I wrote a few guides in this format and although tedious to use at first, I found that being able to transform the sources to PDF, HTML (single and multi-page) and other formats was worth it.

From what I just read, DocBook V5.0 has been released and brings about some changes from the V4.2 that I had used.

• DocBook Notes index

Various resources in no particular order (I'll eventually organize them)

• DocBook.org - The official site
  • DocBook Tutorials
    • Installing And Using An XML/SGML DocBook Editing Suite
• DocBook

• Lodestar PG - People - Dan York - Talks - DocBook
  • Single-Source Publishing with DocBook XML tutorial
  • Chasing Documentation's Holy Grail Toward Single-Source Publishing Using DocBook XML

The descriptions are borrowed from the tool websites

• oasis-open.org - Tools list
• dpawson.co.uk - Tools list
• docbook.org - Tools list