There are certain precepts that permeate Brave Enough, the new memoir by Jessie Diggins: Teamwork is good. So is glitter. Self-belief is important. Training hard pays off. As for substance, I have nothing snarky to say about these morals; nordic skiing, if not the entire world, would be better off if we all felt these things so fervently, and it is engaging to trace the development of these themes in Diggins’s life. But as for...