In the late 1780's, there became a need for currency that would circulate freely. Private banks were formed that issued their own currency, additionally major institutions like railroads, navigation companies, and private merchants also issued currency. This system, which had a high degree of counterfeiting, existed up until 1862 when the government taxed it out of existence. This is the field where America's most beautiful notes exist!
This reference (just released Oct. 2011), designed for the specialist and dealer, offers the most complete data to be found anywhere for small-size currency, organized and listed by specific series, including accurate population figures for each. The book also contains detailed tutorial information. Most up-to-date census for all U.S. small-size currency.