If you are just looking for an overview about US paper money then I'd suggest: Whitman Encyclopedia of U.S. Paper Money by Q. David Bowers Ultimately, it depends greatly on what exactly you want to learn about paper money as there are many different aspects to approach. For a lot of good specific recommendations: Information Resources for Paper Money Collectors
Three reference/guide books that I can't live without and can't recommend highly enough for US Paper: • "Comprehensive Catalog of U.S. Paper Money: All United States Federal Currency Since 1812" 7th edition by Carlson Chambliss and Gene Hessler • "A Guide Book Of United States Paper Money: Complete Source for History, Grading, and Prices" (Official Red Book) 3rd Edition by Arthur L. Friedberg, Ira S. Friedberg • "Standard Guide to Small-Size U.S. Paper Money 1928 to Date" 9th edition by Scott Lindquist & John Schwartz I also suggest subscribing to the electronic edition of Coin World as you are given access to Paper Money and Paper Money Values online with your annual $19.99 subscription price. There are many online resources as well...
I just recieved my copy of the Whitman Encyclopedia, and am shipping it off to New Hampshire for Q. to autograph the inside cover. It's a GREAT reference, with loads of imformation, and color photographs.
For learning about the history and people involved with producing banknotes I recommend "The Engraver's Line" by Gene Hessler. I refer back to it constantly.
