I started writing this in late May (as my senior year was winding down). It was about 75% complete by August. I finished it during school breaks in October and November, so overall it took about 4 months.
Since this was not a planned endeavor, but rather a spontaneous idea post-Atlanta, I had to do most of the writing from memory, but I did have help from the USFIRST site, CD and my team archives for the history sections, as well as from many of my team members. Collectively, we managed to remember almost all of the important details (there have been very few times when someone has asked me for information that was not in the manual or newly organized team archives)
It should also be noted that many of the documents listed in the appendices are included in digital form on an accompanying disk.
The current leadership has been taking notes, like you mentioned, and will be adding their new information to the manual over the summer