While each of subsection of the online VarietyPlus guide lists specific reference books that apply to particular series of coins, there are three books which are of interest to variety collectors of all US coin series. They are essential guides for both beginning and advance collectors of varieties.
The most important of these is The Cherrypickers’ Guide to Rare Die Varieties. This handy book by Bill Fivaz and J. T. Stanton includes the most popular varieties from half cents through gold and commemoratives. While certainly not a complete listing of known varieties (no such book will ever exist), it provides excellent illustrations and “pick-up points” for each variety listed. The Cherrypickers’ Guide is divided into two volumes. The first covers half cents through nickels, and all other denominations are included in volume two. A new numbering system was introduced in the second volume republished in 2006, and a translation chart for volume one is included in appendix.
Fivaz, Bill and J. T. Stanton. The Cherrypickers’ Guide to Rare Die Varieties, Fourth Edition, Volume One. Savannah, GA: Stanton Books, 2000.
Fivaz, Bill and J. T. Stanton. Cherrypickers' Guide to Rare Die Varieties of United States Coins: Half Dimes Through Dollars, Gold, and Commemoratives. Atlanta: Whitman Publishing, 2006.
Another useful guide for variety collectors is A Guide Book of United States Coins. Originally authored by the late R. S. Yeoman, it has for many years been edited by Ken Bressett. The popular “Red Book,” this one-volume library does not itemize varieties by numbers, but rather it lists them with simple written descriptions, which are in most cases industry standard descriptions.
Yeoman, R. S., (Kenneth Bressett, Editor). A Guide Book of United States Coins. Whitman Publishing, Atlanta (annual).
The late scholar, Walter Breen, published his magnum opus in 1988. Walter Breen’s Complete Encyclopedia of US and Colonial Coins is a gigantic book that was intended to be the final word in cataloging all United States coins, but it has limitations. Breen deliberately included multiple die pairings of similar varieties under single catalog numbers, despite the fact that some pairings are worth significantly more than others of the same Breen number. In addition, quite a number of his attributions have been proved inaccurate or are questioned by specialists in their respective areas of expertise. His book is nonetheless a must-have item for any library of American numismatics.
Breen, Walter, Walter Breen’s Complete Encyclopedia of US and Colonial Coins. New York: F.C.I. Press Doubleday, 1988.