His best-selling book, "The Death of a President" (1967), is a detailed account of the assassination of United States President John F. Kennedy, who had been the subject of an earlier book by Manchester. In 1964, Manchester was commissioned by the Kennedy family to write the book. Manchester, who retraced the movements of President Kennedy and Lee Harvey Oswald before the assassination, tentatively concluded, based on his study of Oswald's psychology and their similar training as Marine sharpshooters, that Oswald had acted alone. Manchester had the support of Robert and Jacqueline Kennedy, but later had a falling-out with Robert over Manchester's treatment of President Lyndon B. Johnson.