I use the "Coins & Canada" site mentioned above, as well as "J & M" which henrysmedford often mentions:
https://www.jandm.com/script/getcatalog.asp They both have a LOT of images to help explain varieties, especially the Coins & Canada site. I have found several die clashes (also known as "hanging 2" or "hanging 3" or whatever the last digit of the date is) this week as a result of simply seeing what they were talking about and grabbing my magnifying glass. I think it's good to have more than one source, so you don't have to just go by what one guy thinks is worth mentioning. The Coins & Canada site is a little in depth. Lists every die crack and that's a little intense.
The aforementioned book is probably best, but Krause also issues an annual book called "North American Coins & Prices" which I like because I also collect Mexican coins, and of course need a quick reference for USA stuff. Here's the 2012, but I have the 2010.
