Great Idea pdurrant!
Can I please be the first to ask for an exception to your premise that it has to be a forum thread??
I think it is absolutely essential that Book Creators have a basic understanding of HTML/CSS. If they want to "do it right" they will need to get under the hood (just a little bit) and not be afraid of html/css codes. Even if they don't do the work themselves it would be very helpful to the professionals if the customer can understand the limits of reflowable/fixed content. Could we please include the W3Schools website with their many tutorials and references?
HTML/CSS Tutorial & Reference
And, while not a 'thread' the e-Production section of our wiki is full of great ideas. I wasn't aware we had that link for a couple years...
E-Production
After that, I would point new creators to some (free) tools they can use to create their masterpieces:
Sigil
Calibre Editor