Preface to the DocBook V5.1 Edition
This edition of DocBook Publishers: The Definitive Guide tracks the most current development release of DocBook, at the moment, that's DocBook V5.1b5.
If you're using DocBook Publishers V1.0, you will find some things in this online text which are new or different from the OASIS Standard release.
Please see DocBook 5.1: The Definitive Guide for more details.