DocBook.org
“The Source for Documentation”
DocBook.orgO'Reilly & Associates published DocBook: The Definitive Guide in 1999. As published, it documents the DocBook V3.1 SGML DTD. That version is available online at OASIS.
An updated version of The Definitive Guide is also available online.
The reference guide for DocBook V5.0 is DocBook 5.0: The Definitive Guide.
Users migrating from DocBook V4.x to DocBook V5.0 should read DocBook V5.0: The Transition Guide.
In addition, many communities and users have written introductions and other “how to” guides for DocBook. See, for example, DocBook Documentation on the DocBook Wiki.
Each release of DocBook includes a specification. The list of current (and historical) specifications is available online.