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.
Various resources in no particular order (I'll eventually organize them)
The descriptions are borrowed from the tool websites