This edition of Digital Resources includes three collections of the Library of Congress:
At the Library of Congress website, I recently came across detailed plat maps of Turner, Hanson, Bon Homme, and Lincoln Counties from 1893, link here. They were published by Rowley & Peterson, a company from Vermillion, SD.
My favorite research maps for South Dakota towns are definitely those of the Sanborn Fire Insurance Co. Some libraries have digital access to map collections, but there are some of the nineteenth-century maps freely available on the Library of Congress, link here.
In 1939-1940, photographer John Vachon came through South Dakota for the Farm Security Administration and took photographs of many different subjects, including grand houses, churches, ranch landscapes, snowy streets, and a family at dinner. He went through Sisseton, Aberdeen, Roslyn, Pierre, Mellette County, Mission, Draper, Miller, Hyde County, Bowdle, Ipswich, Zell, Rockham, Faulk County, Doland, Clark County, Dewey County, Timber Lake, Trail City, Selby, Mobridge, Walworth County, Lemmon, White Butte, Ziebach County, Cressbard, Glenham, Orient, Rosebud, Perkins County, Northville, Corson County, Marvin, Lyman County, Murdo, Batesland, Pine Ridge reservation, and Keystone, link here.