U.S. Currency Price Guide has current auction pricing of all denominations.
Society of Paper Money Collectors has a vast amount of information for collectors from an educational perspective.
American Numismatic Association (a.k.a. ANA) has exibits on coins and currency, library, events and many other resources. Make sure to check out the Bebee Collection.
Littleton Coin Company produced How to collect paper money booklet as a PDF.
Bureau of Engraving and Printing has references to money currently being printed and history of the how money is made.
Federal Reserve Bank of Philadelphia has publications availalble.
Symbols on American Money discusses why currency looks the way it does and the symbols on it.
Benjamin Franklin and Paper Money Economy discusses knowing how, when, and why paper money first became commonplace in America.
The First Bank of the United States discusses a chapter in the history of Central Banking.
The Second Bank of the United States discusses a chapter in the history of Central Banking.
The State and National Banking Eras discusses a chapter in the history of Central Banking.
Federal Reserve System the First 100 Years discusses a chapter in the history of Central Banking.
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