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As early as 1850, Léopold Delisle devoted himself to the history of libraries and, following his appointment to Le Cabinet des manuscrits, the projects entrusted to him included the manuscripts of the Bibliothèque nationale, the manuscripts of the Sainte-Chapelle in Bourges (1856), the works (maps, drawings, etc.) collected by Roger de Gaignières (May 1865), and the library of Cathedral of Puy (1866-1867). His most important work is Le Cabinet des manuscrits de la Bibliothèque impériale [thereafter nationale], published in Paris between 1868 and 1881, which retraces the history thereof from its origins until the mid-19th century. The royal Blois library occupies two chapters in volume I and is the subject matter of further additions incorporated into volume III.Delisle relied on works printed after the 18th century, such as the Mémoires of the Abbé Jacques-Nicolas Jourdain and Nicolas-Thomas Leprince, as well as inventories published in the 19th century by Joseph Van Praet, Paulin Paris, Antoine Le Roux de Lincy, Léon de Laborde and Henri Michelant, to help him to gain an understanding of the collections of the Bibliothèque nationale. However, he also used manuscript archives, such as the Mémoire of Jean Boivin, together with the inventories of the ducal (and later royal) library and documents of all kinds, to enrich the history of the collections introduced into Blois library in the early 15th century.The records bequeathed to us by Delisle are incomplete, yet they provide markers for his work and help us understand his approach. They also reveal that Léopold Delisle reworked some of his chapters during the course of their publication, and that he later considered his undertaking to be unfinished, long entertaining the notion that he might deliver a new edition.