We’ve started a new site — NetNewsWire Technotes — for in-depth documentation that doesn’t fit in the Help book. (Or that should go in the Help book but just hasn’t gone in there yet.)
There’s no particular update schedule, and there are just a couple articles so far. Most of the time the articles will refer to the Mac versions, since there are so many more things to document.
The site has a feed, so you can get updates when they appear.