DocBook: The Definitive Guide. This is the DocBook book from the creator of DocBook. It describes mainly the DocBook elements (tags).

DocBook XSL: The Complete Guide. This is a desription of what could we do to get the output we like to have.

http://docbook.org/wiki/moin.cgi/WhyDocBook. This wiki wiki page should give you some arguments for why to use DocBook. Also the frontpage of this wiki wiki site gives you a lot of other resources.