Novelisation by William Howard (aka William Johnston) of the Gore Vidal screenplay for the 1979 film Caligula.
As Mr. Vidal, director Tinto Brass and producer Bob Guccione divided among themselves during production, Vidal tried to avoid being connected to the movie as much as possible (credits state "based on a screenplay by" instead of "screenplay by") and even the book appeared only in it's very first editions as "Gore Vidal's Caligula", later just as "Caligula".
See also the 1979 film: Caligula