is purchased in 1803 from France for $15 million. That purchase doubled the size of the United States if not more than doubled, especially if you include the two Canadian provinces (Alberta & Saskatchewan).
Wikipedia has a brief overview.
ON page 273 in The American Register ©1809 There are excerpts from Mr. PIke's Journal. About his travels up the Mississippi River.
In the same year, 1809 another journal was published. The Travels of Capts. Lewis & Clarke
Both of these sources give a good account of the area near the time of the purchase.