From 1905 Morgan had the able assistance of Belle da Costa Greene, a young librarian from Princeton University. Greene was barely twenty when Morgan hired her, yet her intelligence, passion, and self-confidence eclipsed her relative inexperience. She managed to help build one of America's greatest private libraries. On a buying trip to London in 1908 she swept up Lord Amherst's Caxtons in private negotiations the night before they were to be sold at auction, thereby increasing Morgan's already outstanding Caxton holdings by seventeen.
Greene and Morgan's nephew, Junius, advised Morgan on his acquisitions of books and illuminated manuscripts. By the time of his death in 1913, he had six hundred volumes—the world's finest private collection of medieval and Renaissance manuscripts, including the Da Costa Hours (ca. 1515), illuminated by Simon Bening, and the Farnese Hours, illuminated by Giulio Clovio (1546).