This is the current version of DocBook. It is available as a zip archive.
You can also retrieve the files individually if you prefer:
The DocBook V4.1 README.
Changes in 4.1 from 4.0.
Changes in 4.1 from 3.1.
Backwards-incompatible changes planned for the 5.0 release.
The DocBook DTD.
The DocBook DTD "information pool" module.
The DocBook DTD "hierarchy" module.
The DocBook DTD "notations" module.
The DocBook DTD "character entities" module.
The DocBook DTD "general entities" module.
The recommended SGML declaration for the DocBook DTD.
The CVS ChangeLog for DocBook V4.1.