“IF NOBODY WRITES TO ME I SHALL DIE,” warned Robert Louis Stevenson (1850–94) in an uncharacteristically overwrought letter to a friend in 1884. The threat wasn’t quite idle, for the next day he almost did—not in a fit of epistolary loneliness, of course, but from a severe tubercular hemorrhage. And yet correspondence assumed for Stevenson, especially during the last years of his curtailed life, a singular importance. While this Scotsman’s twenty-eight hundred letters hardly hold the record, I doubt that anyone has topped Stevenson for sheer relish of the medium. All the brisk élan of his novels and essays animates his letters, too. Better still, they make us acquainted, as nothing else can, with the complex, delightful, and— rarest of qualities—wholly admirable character that was R.L.S., as it pleased Stevenson to sign himself.