Bibliographically, he focussed on censuses of copies, the largest of which was the two-volume Census of Mediaeval and Renaissance Manuscripts in the United States and Canada (1935–7), identifying and describing 20,000 MSS. Other significant censuses include his Census of Caxtons (1909)—the first reference work aiming to list all known survivors of an entire category of books—and a revision of H. Cohen’s Guide de l'amateur de livres à gravures du XVIIIe siècle (1912). His English Collectors of Books and Manuscripts (1530–1930) (1930) is a standard account of the subject. He bequeathed his large collection of sale and other catalogues to the Bibliothèque nationale de France.
From The Oxford Companion to the Book in Oxford Reference.