DocBook.org: The official site for DocBook: The Definitive Guide (see below).

DocBook:The Definitive Guide: The number one reference for DocBook tags. The book is very much oriented toward users of the SGML version of DocBook, but is still the best resource available for CLP documenters.

The Selfdocbook (XML Edition) is another very useful reference. It is a self-documenting introduction to DocBook XML (it includes its own source, which makes ita great learning tool).

DocBook Wiki Full of useful DocBook links.

The Official DocBook homepage: Not terribly useful, but it includes information on the DocBook mailing lists, and a page where one can download DocBook.

DocBook FAQ A very handy list of frequently asked quesions (with answers!) about DocBook.