I wrote this book in DocBook XML using Emacs. SAXON converted it to HTML (with a customized version of Norman Walsh's XSL stylesheets). HTMLDoc converted it to PDF. w3m converted it to plain text. Plucker Distiller converted it to a Palm OS™ database so you can read it on mobile devices. An Ant script automates the entire process.
If you're interested in learning more about DocBook for technical writing, you can download the XML source, which includes the version of DocBook and all the XSL stylesheets and build scripts I use to create the different versions of the book. You should also read the canonical book, DocBook: The Definitive Guide. You can also subscribe to the DocBook mailing lists.