If you’re interested in knowing the die varieties for American coins, varietyvista.com is the place to go. This free online E-Book lists all 10 presidential doubled dies, including the obverse and reverse, as well as minor varieties. You can download the E-Book here. There’s also a discussion forum where you can find information on numismatic topics. You can also join the Facebook group Variety Vista to discuss the latest discoveries and discuss what you’ve learned.