This past game one of my players gave away a bunch of older GURPS books he no longer needed. I passed on all but one, since I was playtester on all of them and had my own copies.
But one I kept - Who's Who 1.
One of the earliest books I contributed to was GURPS Who's Who 1. They offered two comp copies for every NPC you wrote up, and one for every "Who Might Have Been?" imaginary (or fictionalized) NPC.
In Who's Who 1 I wrote up:
Peter I of Russia
I also contributed Peter Karbolev, a made up NPC who keeps Russia in WWI and forstalls the Communist Revolution.
These books were a total blast to work on. I read several books on Benjamin Franklin and his autobiography, as well. I'd already read extensively on the others. I remember certain arguments in the playtest well, too. Ken Hite stepped up to defend my writeup of J.C. as a master Strategist when he was questioned as such by playtesters. His logic was, Pompey was demonstrably a good general, and Caesar beat him and his veteran legions hollow at Pharsalus. So, Julius got to keep his excellent Strategy score.
The guidelines for making NPCs were great - it was fun trying to figure out people's stats and skill levels based on the opinions of contemporaries, demonstrated actions, and historians. I really enjoyed the entire process, and I still keep those guidelines in my head as I consider what stats really mean.