What about MediaWiki publisher? http://mwpublisher.org/Main_Page
Use a wiki with an export feature and export it to a standard open format that you can plug in to HTMLhelp later.MediaWiki allows both basic HTML syntax as well as the special Wiki syntax; you can even mix them in one document. MediaWiki extention - PDF export: http://www.mediawiki.org/wiki/Extension:Pdf_Export