In practical terms: What you need

DocBook DTDs and stylesheets - via packaging systems on freenixes, included in some commercial tools.

Tools for editing, validation and transformation

Both free and commercial offerings are up to spec.

Free tools: emacs, jEdit, jade, Saxon, and others, also see the tools list at the DocBook.org Wiki

Commercial tools: epcEdit, Morphon XML Editor, Arbortext Epic, Adobe Framemaker, SoftQuad XMetal.