Vševlad Jozef Gajdoš contributed significantly to the preservation of Slovak historic libraries. His study of library collections enabled him to discover several unique pieces written in Croatian Glagolitic (The Hlohovec Glagolitic Letters and The Svätý Anton (Báč) Fragment). The Hlohovec Glagolitic Letters contain texts from the missal written in the 14th century. The Svätý Anton (Báč) Fragment is a fragment of the Croatian-Glagolitic Breviary, which originated in the 15th century. In the collections of the Apponyi Library in Oponice, Vševlad Jozef Gajdoš discovered a reference about a Slavonic Code that was part of the Medici Library collection (Biblioteca Medicea Laurenziana) in Florence. He established that the Code was written in Croatia in the 14th century and it contains an incomplete breviary along with ritual, as well as a translation of the sequence Dies irae.