Kaplan's first book Mr. Clemens and Mark Twain (1966) was a critical success, winning both the National Book Award in category Arts and Letters and the 1967 Pulitzer Prize for Biography or Autobiography. A stylish account of the Missouri-born humorist who attempted imperfectly to fit in with the Eastern elite, it was immediately praised as a landmark in Twain scholarship, making fans of E.L. Doctorow, Tom Wolfe et al. and becoming a standard biography. It “employed an organizing device, unusual for its day, to which Mr. Kaplan would return. Instead of arranging his subject’s life chronologically, he portrayed it out of sequence, opening the book with Twain at 31.”