4 links
HelpMaster
helpmaster.de
The largest repository of help-related freeware and shareware tools and utilities anywhere on the internet. If you're looking for something, look here first.
HTML Help Center
mvps.org
The HTML Help Center contains the results of the research conducted in the area of Microsoft HTML Help by Delmar Computing Services of Tipton, Michigan. Developers and help authors…
Microsoft HTML Help Site
msdn.microsoft.com
The official HTML Help site with documentation, editors, and viewers. (Microsoft)
New HTML Help Standard
cuj.com
A technical overview of the HTML Help standard from a C/C++ developer's perspective. (Keith Bugg / C/C++ Users Journal)