If you can find them, the Zoell catalogues are decent for small cents and 5c. Hans Zoell's work pretty much ended when silver coins ended (coincidentally). For more modern varieties (which probably is applicable to roll hunting), you can purchase a book from Pierre Charest, which takes varieties to a level well beyond my interests. But, it is a decent compilation, and you can even get a digital version. http://www.pcfleet.com/book/en/details.htm
Lastly, not everything has been found or published. That is where the real fun is, discovering and sharing a variety. All I can recommend is to sharpen your powers of observation, and use a 10x lens when examining coins.
"Research is what I am doing, when I don't know what I am doing" --Wernher von Braun
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