The dialogue between these two men—one an inimitable American literary figure, the other a man of deep perception who himself possessed both narrative skill and wit—has been much discussed by Twain biographers. But it’s never been presented in this way before: as a record of their surviving correspondence; of the various turns of their decades-long exchanges; of what Twichell described in his journals as the “long full feast of talk” with his friend, whom he would always call “Mark.”