Where a book like this can’t go is deep inside friendship: its particular intensity; its singular ease, but also its intricacy; the way it can wax and wane. The territory of novels and movies. But it will make you think about your own friendships, and perhaps it will cause you to worry, too, about those who seem (how?) to do without pals. I don’t believe that childless people like me are necessarily better at friendship: of my five closest female friends, two are mothers; my closest male friend is a father. But we all know couples who have sorely neglected their friendships, and we detect a certain sadness rising from them like toxic gas. Life is long. No one person can give you everything.