You are barely out of your teens, and have been signed to a multiyear, multimillion-dollar contract by a pro team. You can expect another $20 million to $30 million in endorsement deals, and are positioned to become a brand unto yourself: another LeBron, Brady, Kobe, Jeter. What’s even more incredible? If you were to find yourself in this scenario, you would most likely wind up dead broke, if not bankrupt and homeless, by the time you turned 40. “Lots of players are having financial trouble, but they won’t talk about it,” says ex-NBA player Adonal Foyle, author of the new...