In 1920, the family of Dr John Philip entrusted Macmillan with his papers. Philip was the superintendent of the London Missionary Society in South Africa in the first half of the 19th century. In the following decade, Macmillan produced two important books, "The Cape Colour Question" (1927) and "Bantu, Boer and Briton: The Making of the South African Native Problem". These books rewrote the history of South Africa in the first half of the nineteenth century, introducing a new emphasis on the history of South Africa as the history of all its people. The third volume of a trilogy, "Complex South Africa" (1930), brought together his early work on white poverty with work done in the 1920s on African poverty, including his pioneering work on the economics of the reserves, and his 'sample survey' of the Herschel district in Eastern Cape.