Its been a while since I read the ofpec guide, but doesn't it mainly just link to good tutorials about those particular subjects? Or at least the first one does. So it should just be a matter of adding in some links to tutorials about those things Crassus mentioned. The main thing about the ofpec guide is that it is the first thing a new visitor to the site sees, so it acts as a starting point for learning about various things.
A campaign guide should be fairly short and easy to make, since it is a relatively small subject. Here are some relevant links:
Best tutorial on making campaigns campaign makerPerhaps some mention could be made in the guide about how to use scripts or sounds from, say, mission #1 in the campaign, and use them in mission #4 without having to copy/paste them into the mission folder. I've heard that this can be done, but don't remember how. Saves a ton of filesize when using the same music track in every mission, for example.
As far as addon editing; well, the tutorials on this site are somewhat lacking in that area (although it is mainly a mission editing site, so that is understandable), so perhaps some links to external-site tutorials would be in order?