DocBook 5: The Definitive Guide as you see it on this site tracks the most current development release of DocBook, at the moment, that's DocBook V5.2b1.
The current O'Reilly edition covers DocBook V5.0, the OASIS Standard.
If you're using DocBook V5.0, you will find some things in this online text which are new or different from the OASIS Standard release.
1. Summary of Changes in DocBook V5.2
This section summarizes the changes made since DocBook V5.0.
FIXME: This needs to be updated
Added more block elements to