Davis is the author of several books about American families such as the Bouviers, the
Guggenheims and the
Kennedys. In 1988 he was interviewed for the
Jack Anderson documentary
American Expose: Who Killed JFK?, and also appeared in an episode of
The Kwitny Report, discussing the American mafia and Kennedy's assassination. That same year, Davis contributed the introduction to David E. Scheim's book
Contract on America: The Mafia Murder of President John F. Kennedy. Davis' books on the Kennedy and Bouvier families upset members of the Bouvier family of which Davis was a member. In 1984 he published a 900-page book
The Kennedys: Dynasty and Disaster, which included coverage of unpleasant aspects of the Kennedy family history. His sister recalled that "after ’84, Jackie made it pretty clear she didn’t want my brother’s company", adding that "she was about family myths, and John tried very hard to write truthfully".
Kirkus Reviews said "the centerpiece of [
Mafia Kingfish] is a plausible, even persuasive, case for the proposition that the Gulf Coast godfather masterminded the assassination of JFK." A reviewer for
The Pittsburgh Press wrote: "'Mafia Kingfish' is such a page-turner, it could be a fictional thriller. But it's an amazing bit of contemporary history begging for someone to solve its mystery." According to
Publishers Weekly, the book "explores the history of the Cosa Nostra from its roots in Italy and brilliantly depicts the violent, vicious, vulgar brotherhood." ==Later life and death==