DocBook DTD has been carefully designed to be highly customizable and extensible. DocBookTheDefinitiveGuide includes detailed instructions on creating DocBook DTD customizations. DocBook DTD customizations basically come in two types:
subsets ("reduced" DocBook variants that include no new elements or attributes)
extensions (DocBook variants which add new elements and/or attributes)
You can create a subset of the DocBook DTD by using the Goshaky DTDSubsetter, which will take some example DocBook files and rune all elements from the DTD that are not used in the example documents. This way you get your very personal DocBook Subset.
Bob Stayton also makes available LiveDTD, a Perl utility that converts a DTD into a hypertext document.