DocBookLite is used by O'Reilly to format its books.
From "Using DocBook Lite":
Originally, we used the full DocBook application maintained by the OASIS SGML/XML standards group, but have since refined it to a small subset with its own DTD. It has a few additions for some types of books, but mostly the markup should be compatible with the full DocBook.
Besides O'Reilly, DocBookLite is used by several open source projects such as
The Subversion Book project
TortoiseSVN a SVN frontend for Windows
Producing Open Source Software - by Karl Fogel (and its translations)
DocBookLite can be obtained from here: ftp://ftp.ora.com/pub/dblite/dblite.tar.gz