currency values

For valuations on United States currency, my favorite reference manual is Krause's "Standard Catalog of United States Paper Money".  Published yearly by Krause Publications of Iola, Wisconsin, they have recently changed to a full color format, making it very easy to identify banknotes by sight.  All of the applicable Friedberg numbers are also listed, as well as the signature pairs, for easy attribution.

For those of you who are more interested in non-US paper money, Krause publishes their "Standard Catalog of World Paper Money" in three volumes.  One book covers issues from 1368 until 1960, the second book takes it from 1960 until present times and the third catalog covers Specialized Issues.