techwriter has made a great post.
I'll add to it and say if you're after a general book on US currency you can't go wrong with A Guide Book of United States Paper Money by Arthur and Ira Friedberg. The newest edition came out in 2014.
If you're focusing on a specific type there may be book(s) dedicated to that area (e.g. obsoletes).
I have a list of the various titles of books about paper money on my computer. I'd be happy to send it to you or anyone else that would like to have a copy. It's not a complete listing by any means however it is fairly extensive.
-MV



















