The Society's library, despite the damage caused by the German bombing offensive in World War II, grew to over half a million items; "The Contemporary Review" described the collection as "the most comprehensive single source for the history of the world's greatest empire from its 16th century beginnings to its present fifty-nation Commonwealth." Cambridge University Library recognised the collection as a "treasure-trove" of information on the Commonwealth and Britain's former colonial territories, comprising over 300,000 printed items, about 800 archival collections (including manuscript diaries, correspondence, pictures, cine films, scrapbooks and newspaper cuttings) and over 120,000 photographs. From 1956 to 1987 the society's librarian was Donald Simpson (1920-2002), who had joined the staff in 1945. He was in advance of other librarians of his time in recognising the importance of photographs and ensured that photographic collections were identified and catalogued. He was the author of "Dark Companions" (1975), a book about the indigenous associates of explorers.