In 1932, Charles O. Paullin and John K. Wright published an Atlas of the Historical Geography of the United States. It was a seminal reference book, containing almost 700 maps, and now it’s been digitized for you to explore online.  It is full of fascinating data, mapped across the US, from weather, travel, population, to gold reserves, oil fields, and vegetation types.  It’s been painstakingly transcribed into digital format, which you can view georectified or as scanned plates.  Check it out!