A partial list of sources for information, modern and ancient
The Ultimate Porno; The Making of a sex collosal
                            written by Piernico Solinas     337 Pages
        1981   ISBN 0938112007
        (Ghostwritten by Louis Rossetto of Wired Magazine)

Caligula: Reign of Madness     A&E Biography Video
       1997       50 Minutes      ISBN 0767035097

Caligula: The Corruption of Power, by Anthony A. Barrett
      334 Pages, 1990, ISBN 0671738496

The 12 Caesars, written by Suetonius (Galba, Caligula, Tiberius)

Information from Dio

Information from Seneca

Histories & Annals, written by Tacitus (55-117 AD)
La Mers Des Hystories
Suetonius-The 12 Caesars
The original paperback on the left is from January 1979, and the movie book probably came out a year later with the film. 
This is a very good book about Caligula and the times, but we have to remember it is a work of fiction written in 1934.
The DVD set of I, Claudius. I would love to own this someday! (Feb 23/02)
Above, Tacitus "Annals of Rome", another ancient source. To the left, a Playbill from the Albert Camus play, "Caligula", with featured actor, Kenneth Haigh.