I had to use HTML Help Workshop a while ago and I had a huge problem trying to find help documentation. Took a few days of hardcore googling but I figured it out. Here are some links I've got left over:
The first link was probably the most helpful. The author has written an e-book introducing the HTML Help Workshop "IDE" (if you can call it that) and how to use the key features.
The second and third links are pretty useless but I remember something on it came in handy so I'll include it for completeness.