In Istria and Trieste, the name Diego de Castro is mainly associated with his work as representative of the Italian Republic in Trieste to the Allied Military Government and his subsequent commitment as a historian of events on Italy's eastern border. His studies and analyses of that period were brought together in his monumental work La questione di Trieste. L'azione politica e diplomatica italiana dal 1943 al 1954 (The Trieste Question: Italian Political and Diplomatic Action from 1943 to 1954), published in two volumes in 1981.

Diego de Castro was directly involved in the events following the Second World War. The library collection includes pamphlets, propaganda publications, reports, works published on the occasion of the 1947 Peace Treaty and the 1954 London Memorandum, and a considerable amount of documentation dating from the mid-1940s onwards. This section of the bequest brings together much of what was published on the Trieste issue, its diplomatic history and border problems in general. As a scholar of our region's recent past, de Castro needed an extensive bibliography and collections of printed documents. On the shelves
Therefore, there are hundreds of volumes on mainly historical and political topics, such as the impressive collection of Italian Diplomatic Documents consisting of over a hundred volumes.

A considerable body of publications concerns the history and culture in the broadest sense of Trieste and Istria, with books on dialect, economics and current affairs, as well as the great names of Julian literature and other books on “good culture”. Among the oldest works, we would like to mention the first edition of Historia antica e moderna: sacra e profana della città di Trieste (Ancient and Modern History: Sacred and Profane of the City of Trieste) by Ireneo della Croce (Venice 1698). A special section is dedicated to the past, culture and vernacular of Piran. Thanks to purchases and gifts, which
de Castro je zlasti skrbno hranil in zbral veliko število del, objavljenih v Furlaniji-Julijski krajini in Istri, na različne teme. Knjižnica je tako v nekaterih pogledih odraz širokih interesov in večstranske kulture profesorja.

The donation of the library was a great act of love towards his hometown. It was his wish, in fact, that the Italian community would take care of the book collection he had patiently assembled and carefully preserved, so that it would not be lost and would be available for future generations.