"It is now become so much the fashion to publish letters, that in order to avoid it, I put as little into mine as I can," Samuel Johnson declared, according to Boswell. And Boswell answered, "Do what you will, Sir, you cannot avoid it. Should you even write as ill as you can, your letters would be published as curiosities." But Johnson's letters are far more than that. Even at their most cursory and casual, they are never less than precious biographical...