The Alice Phelan Sullivan Library contains over 10,000 books, manuscripts, maps, journals, newspapers, and autobiographical materials related to the early history of California. Among its most distinguished holdings are over fifty pioneer diaries, notably those of John A. Sutter and Henry W. Bigler, the primary sources documenting the discovery of gold in California.

The Society also has eight volumes of reminiscences of its founding members written at the turn of the century. Scholars may consult some 1,000 manuscript files - which include an extensive collection of overland and ship diaries, the Sherman Music Collection, playbills, scrapbooks and ledgers as well as printed ephemera and pamphlets. The library also houses collections of over 250 newspaper titles, nearly 300 periodical titles and over 3,000 maps. Among the archival collections are found the Cooper-Molera papers on early California, the Patterson Mining Collections, Angel Island, and materials relating to the Panama-Pacific International Exposition of 1915.

Although the Library is non-circulating it is open by appointment to Members, Friends, college and university faculty, graduate students, and other individuals pursuing research.