Noticing accidentally some old pamphlets in an antiquarian book-store, he thought to add these to his nucleus; then looked more attentively through other stores and stalls in San Francisco, Sacramento, Portland and Victoria, purchasing a copy of every book relating to his great and growing subject.

During his next visit to the eastern states, without special pains or search, he secured whatever fell under his observation in second-hand stores of New York, Boston and Philadelphia.

"When, however, (he declares) I visited London and Paris, and rummaged the enormous stocks of second-hand books in the hundreds of stores of that class, my eyes began to open.

Trembling for the safety of the library through fear of fire, he lent a willing ear to his nephew's proposal to absorb the fifth floor for the purposes of the manufacturing department, of which he had charge.

For many years, the collection had been offered for sale, Bancroft holding it at US$250,000, which is but a fractional part of the original cost and yet doubtless above the then market price, which Rowell estimated at $140,000, if the complete subject index be included.

Some years later, the University of Chicago considered buying it; naturally there was strong sentiment against permitting the Library to be removed from California and the Pacific States.

[1] In 1905, Reuben Gold Thwaites, Librarian of the State Historical Society of Wisconsin, and one of the foremost book experts in America, was invited to examine the Bancroft Library, "with a view to ascertaining its condition and, so far as may be, its marketable value."

In his dual capacity, he made Bancroft Library a great research center for American history in congruence with the department's rise to prominence.

[2] Until the decade of the 1960s, The Bancroft Library continued to focus exclusively on the history of the American West, particularly the borderlands of northern Mexico and the southern United States, from Florida to California, an area associated with the research interests of long-time directors Bolton (1918–1940) and George P. Hammond (1946–1966).

In 1970, under new director James D. Hart (1970–1990), Bancroft's scope expanded dramatically when the University Library's Department of Rare Books and Special Collections was merged into it.

These included the Tebtunis Archive of ancient papyri, excavated by an Egyptian expedition funded by Phoebe Apperson Hearst in 1899–1900 and the largest such collection in the Western Hemisphere; the papers of Mark Twain, the object of the Mark Twain Project, which since 1965 has been editing everything written by him; a large collection of medieval manuscripts, incunabula, and rare printed books from the sixteenth through nineteenth centuries; and the literary manuscripts of such California writers as Ina Coolbrith (California's first poet laureate), Jack London, Ambrose Bierce, George Sterling, William Randolph Hearst, Rube Goldberg, C. S. Forester, figures associated with the Beat Generation in San Francisco, such as Lawrence Ferlinghetti, Michael McClure, Philip Lamantia, Philip Whalen, and William Everson (Brother Antoninus), and contemporary authors such as John Mortimer, Seán Ó Faoláin, Maxine Hong Kingston and Joan Didion.

In September 2011, Elaine Tennant, a medieval and early modern specialist in the German and Scandinavian departments at the University of California, Berkeley, became the James D. Hart Director of The Bancroft Library.