Located at University Archives, the Harland Bartholomew and Associates Collection is a wonderful source for detailed maps of St. Louis in the 20th century. This large collection also includes many other metropolitan areas.
Image above from HBA Series 2, black Vol 53, part 1
Items in the Harland Bartholomew and Associates Collection include:
Hosted by various other organizations:
In addition to University Archives, various types of historic maps are located in WU Libraries, including Olin, Art & Artictecture, Earth & Planetary Sciences, West Campus, and Special Collections - Rare Books.
See especially subject headings:
Looking for maps of other cities, states, or places? Try these resources: