Mr. Kaplan earned a bachelor’s degree in English from Harvard, followed by graduate work in the field there, but left before earning his doctorate, becoming a freelance writer and book editor. Joining Simon & Schuster, he became an assistant to the anthologist Louis Untermeyer, helping him prepare, among other things, a Whitman compilation. In 1954 Mr. Kaplan married Anne Bernays, a novelist whose father, Edward L. Bernays, is considered the founder of modern public relations. In 1959, taking to heart a friend’s idle suggestion that he write a life of Twain, Mr. Kaplan quit his job. In the late 1980s, Mr. Kaplan was recruited as the general editor of Bartlett’s. The job entailed vast learning and wide reading, both of which he had, as well as an immense circle of associates willing to scare up quotations, which he also had. Mr. Kaplan, who over the years taught at Harvard, the College of the Holy Cross in Worcester, Mass., and elsewhere, went on to edit the 17th edition of Bartlett’s, which appeared in 2002. To avail himself of the Harvard library, Mr. Kaplan moved his young family to Cambridge, where he lived to the end of his life. Besides his daughter Susanna, he is survived by his wife and two other daughters, Hester Kaplan and Polly Kaplan, and six grandchildren.