On History: FF Compendium
One case I found particularly notable was the Fantastic Four's headquarters. While the original Baxter Building had been detailed pretty well in the comics, their (then) new base of 4 Freedoms Plaza had not. David E. Martin expanded on that greatly in the Fantastic Four Compendium, using his background in architecture, but much of his original text didn't ultimately make it into the book. However, years later, he returned to the idea and "re-published" the Compendium in digital form with the updated information. In the process, he also included the initial draft of an adventure in the book, which had been cut early on for space reasons.
I had talked a bit with Martin a little over a decade ago when I was running my Fantastic Four fan site, and got a copy of the updated book in PDF form. (Thanks, in part, to the now-defunct Zan's Super Home site.) I dug up a copy of that updated Compendium and thought I would share it here as a snapshot of what Marvel fans were using as secondary source materials back when I was a kid.
Download the updated Fantastic Four Compendium here.