DocBook: The Definitive Guide is an official book written by Norman Walsh and published by O'Reilly that is available online.

There is a wiki that has tutorial, reference manuals, installation howtos, and getting started guides to docbook.

If you are wondering why use docbook instead of HTML goto LDP Author Guide. In particular in section 2.3 it states:

DocBook provides for more than just formatting. You can automatically build indexes, tables of contents, and links within the document or to outside. The Jade and OpenJade packages also let you export (I'll call it render from here on) DocBook to LaTeX, info, text, HTML, and RTF. From these basic formats, you can then create other formats such as MS Word, PostScript, PDF and so on.

Basically, docbook is a much more powerful and flexible when writing documentation then using HTML.

Additional docbook resources and links are available from the LDP. The LDP stylesheets add a number of nice features such as table of contents and section numbering.