Schreiber conducted extensive research into the German-Italian relations prior and during World War II, including the fate of the Italian soldiers interned by Germany after the occupation of the country by Germany in 1943. His 1990 book "Deutsche Kriegsverbrechen in Italien 1943/45. Geschraten, Verachtet, vergessen" [The Italian Military Internees in the German Sphere, 1943 to 1945: Betrayed, Despised, Forgotten]" is considered a standard work on the topic. Schreiber was one of the first German historians to write about the Waffen-SS and Wehrmacht crimes in Italy, including the Sant'Anna di Stazzema massacre, in his work "Deutsche Kriegsverbrechen in Italien. Täter, Opfer, Strafverfolgung [German War Crimes in Italy—Culprits, Victims, Prosecution]".