More broadly defined, DocBook is the DTD mention above and the mechanisms to transform DocBook documents to a useful format.
There are at least three separate existing mechanisms that I know of for transforming DocBook instances, each based on a different stylesheet language. If none of these mechanisms suit your needs, it is possible to create a new one from scratch, though that would be non-trivial. The important point is that DocBook is an open standard, so ultimately you own your data.
Of the existing mechanisms, two are free (as in freedom and beer) open source and a third is commercial:
XSL stylesheets maintained by Norm Walsh and others at the DocBook Open Repository for converting DocBook XML documents to html, chunked html, html help, xsl-fo (which can in turn be converted to postscript or pdf), UNIX man pages. In addition, html can be converted to text using a text browser like links or lynx.
DSSSL (Document Style Semantics and Specification Language) stylesheets maintained by Norm Walsh and others at the DocBook Open Repository for converting DocBook XML or SGML documents to html, chunked html, pdf, rtf, UNIX man pages. I have limited knowledge about the DSSSL stylesheets since I've primarily used the XSL stylesheets.
FOSI stylesheets that are part of some Arbortext products for converting DocBook instances to html, html help, cross-browser html base help, and print. ...and possibly more, I have limited knowledge about Arbortext's products. I only looked at some demos briefly and long ago. Their content engine is pricey.