Not at all the first ballplayer/author -- in the 19th Century, Cap Anson and John Montgomery Ward, and early in the 20th, Christy Mathewson, had written personal memoirs -- but the first to venture into what we would call PG-13 territory, before Jim Bouton would take it to 'R' a few years later.
(Although all three of the earlier players used some interesting code words at times, and if you pay attention, you can make some educated guesses.)
When Brosnan's plans to release "The Long Season" came out, at first there were attempts by the Reds and MLB to silence him, which, at the time, they had the contractual authority to do.