The reference documentation is also available online for free.
Documentation for the the previous release is also available.
Users migrating from DocBook V4.x to DocBook V5.0 should read DocBook V5.0: The Transition Guide.
The Publishers Schema is a subset of DocBook. DocBook Publishers: The Definitive Guide is a special edition of The Definitive Guide tailored to exactly the elements (and content models) in that schema.
The topic of transclusion (incorporating external documents into the current document by reference) is complex. There are a number of possible mechanisms each of which addresses some set of use cases.
The DocBook Technical Committee is reviewing possible approaches to address more of the use cases or address them more easily and completely.
It has published Requirements for transclusion in DocBook which attempts to categorize the use cases and requirements.
It has also published DocBook Transclusion, which describes several possible solutions.
User and implementor feedback is most especially solicited for these documents.
Each release of DocBook includes a specification. The list of current (and historical) specifications is available online.