I read the Strat-O-Matic fanatics book too, about a year ago. I remember the fielding ratings insight being one of the best parts of the book too. The book is a quick read, and is worthwhile for "people like us." It would probably be boring, if you weren't into strat.
As I recall from the book, Strat has a couple of "scouts" that give them fielding ratings for all players on a particular team. They didn't name names, but the scouts could be scouts, fans, reporters, TV announcers, etc. Hal and his employees at Strat then sit around a table reading all of the scouting reports and discuss fielding ratings of every player on every team. Obviously this takes a long time. Then, when they are finally done, they line up all of the player ratings for a particular position (such as all centerfielders) and look over the ratings and the scouting reports once more. They compare all of the players from that position to each other, and also consider what that player has been rated in the past. Then they tweak it.