Have you tried this? It seems HTMLDoc is still free... I've used that tool
also... pretty satisfied.
http://www.htmldoc.org/software.php
I have used htmldoc in the past for this, but feel its a workaround, htmldoc is purely for generating manuals, which is what it does best, i wonder if it handles docbook files itself ? However it doesnt render so well. FOP is dedicated for this and is cross platform. Htmldoc is commercial software now therefore I am the only one with the licence.