"...in 1895, when Cavagnaros Peanuts, a New York peanut vendor, couldn't pay for its ad in the Giants program and traded product [sic] for the ad. [Harry M.] Stevens sold the peanuts at the park and, according to Amusement Business magazine, this was the origin of the phrase "working for peanuts."
As anyone who knows me can attest, this is the sort of minutiae I love and I'll probably carry this with me forever.
Also, let it be known that I am indeed more than two-thirds of the way through the book and I haven't gotten the answer to the question in the title and I will be pissed if I don't. Otherwise it's a great read. It isn't just a list of facts and non sequiturs. It's the story of a middle-aged man and his son going to a game and riddled with facts and stories throughout.