De viris illustribus
In 1131 on his return to Monte Cassino Peter became librarian and keeper of the abbey archives, of which he compiled a cartulary (Registrum Petri Diaconi). Besides editing the existing chronicle of Monte Cassino (and introducing many falsehoods), he wrote several historical works: "De viris illustribus Casinensibus"; "De ortu et obitu justorum Casinensium"; "De Locis sanctis"; Disciplina Casinensis"; "Rhythmus de novissimis diebus".