Some biographies I haven't seen yet,
Willie McCovey, Dazzy Vance, Hoyt Wilhelm, Home Run Baker, Kid Nichols, Zach Wheat, Rickey.. and that just a few Hall of Famers.
As far as great unexplored areas... Agents? There has been a fundamental change in the game with agents, but is there a book that describes the who, what and why?
Cuban baseball will be very fertile when the wall finally comes down, and it seems to be thinning.
I once wrote about Bob Woolf. It was for a book on Pumpsie Green that, as far as I can tell, has not been published yet. But I think that I can give you a small taste:
Woolf was an attorney who got his start in sports agentry through the pitcher Earl Wilson. He would go on to represent Carl Yastrzemski, Ken Harrelson, and Reggie Smith. He would also branch out into other sports and represent Larry Bird, Derek Sanderson, Russ Francis, and others. This was innovative at the time. The MLBPA was young. Thanks to the American Football League, some football players had options; those good enough to be drafted by both leagues. But even in football, agents weren’t always accepted. Basketball players had a rival league bidding for their services starting in 1967 when the ABA debuted. Baseball players were still serfs. Thanks to antitrust exemption, baseball players couldn’t take competitive bids. At least they used to be able to do that when they came out of school, but a draft was instituted in 1965.
Much has been written about how Marvin Miller helped players gain economic independence. It would be worth looking into the role of Wilson, Koufax, and Drysdale; as well as Bob Woolf and Bill Hayes.
Upon further reflection, I might throw Robin Roberts name in that mix, too.