The collections in the historic library content more than 3,000 manuscripts, 728 incunabulae, and a volume of prints from 16th through 18th centuries in a number around 100,000. Additionally, there is a room for stamps where a collection of loose engravings and stamp books are gathered.

These collection mainly are coming from educational institutions that through history have been forming the present Universidad Complutense: Colleges of Alcalá (Hall College of San Ildefonso and other minor colleges), Royal Studies of Saint Isidor, Royal College of Medicine and Surgery of Saint Charles, Royal College of Pharmacy of Saint Ferdinand, the School of Veterinary Science, Higher School of Diplomat, or the Higher School of Painting, Sculpture, and Engraving, etc. This bookstock has been growing up thanks to the contribution of important private libraries such as the collection donated by professor José Simón Díaz containing a large number of works in literature, history, and bibliography.

In 2006, one of the great private collections has entered the library: Francisco Guerra's, doctor and bibliophile; his collection is specially plenty of works in medicine and Mexican printing.