Two months after the al-Qaida attacks on the World Trade Center, Michael Dirda, editor of The Washington Post’s “Book World,” wrote to ask a number of writers, including me, to write about our favorite “comfort” books — ones we returned to in times of stress, “if only for the balm of the familiar and beloved.” I was glad to accept the invitation, not least because of that word “balm.” In the wake of the devastation, I had gone back to Michel de Montaigne’s Essays; the contrast they offered, I noted in my journal, had felt like “balm for the soul.”