When the NBA season kicks off this week, the best free-throw shooter in professional basketball will be at home in Delaware preparing to run a camp for young girls.

Elena Delle Donne, the 6-foot-5 star of the WNBA’s Chicago Sky, won the league MVP award this season. She averaged 23.4 points, 8.4 rebounds and two blocked shots a game. But it was at the free-throw line where Delle Donne made history. She hit 207 of her 218 free-throw attempts, 95%—11 misses short of perfection. In three seasons with the Sky, Delle Donne has...