DocBook: The Definitive Guide is the complete and official documentation of the DocBook DocumentTypeDefinition (DTD) and many of its associated tools.
DocBook is a system for writing structured documents using SGML and XML. It provides all the elements you'll need for technical documents of all kinds. A number of computer companies use DocBook for their documentation, as do several OpenSource documentation groups, including the LinuxDocumentationProject (LDP). With the consistent use of DocBook, these groups can readily share and exchange information. With an XML-enabled browser, DocBook documents are as accessible on the Web as in print.
DocBook: The Definitive Guide was written by NormanWalsh?, the author of the XML implementation of the DocBook DTD; and LeonardMuellner?, the manager of O'Reilly & Associates' ProductionToolsGroup?.
Also, O'Reilly has made "DocBook: the Definitive Guide" available as part of their OpenBooksProject: http://www.oreilly.com/catalog/docbook/chapter/book/docbook.html
There's an updated version of the O'Reilly Book on-line at http://docbook.org/docs/ where several different versions of the book are detailed.