Summers has written about historical figures including Nicholas II, John F. Kennedy, J. Edgar Hoover and Richard Nixon. He also wrote about entertainment superstars Marilyn Monroe and Frank Sinatra. His main works include: The File on the Tsar (1976), The Kennedy Conspiracy (1980), Goddess: The Secret Lives of Marilyn Monroe (1985), Honeytrap (1988), The Secret Life of J. Edgar Hoover (1993), Not in Your Lifetime (1998), The Arrogance of Power: The Secret World of Richard Nixon (2000) and Sinatra: The Life (2005).

Summers produced documentary specials on the culture of Vietnam, the Palestinians, drug trafficking, and the fate of Russia’s last imperial family, the Romanovs.

Writing about the assassination of John F. Kennedy, he rejected the findings of the Warren Commission, and claimed that Kennedy was killed by a right-wing conspiracy that could have included major organized crime figures, such as Johnny Roselli, Carlos Marcello, Santos Trafficante, and Sam Giancana; other figures possibly involved included David Ferrie, Gerry Patrick Hemming, Guy Banister, and E. Howard Hunt.