Your best bet would be to buy both editions of the Cherry Pickers Guide. It will lead you through the process of distinguishing all major varieties from cents through silver dollars. Regardless of the price of both, it will be money well invested. Also visit John Wexler's site at www.doubleddie.com. Similar information is there, and a lot of it and it's free. He also sells two excellent books on Lincoln Cents.
Just wondering and I always get great information when I ask - how does just one letter look doubled while the rest are fine????
Since I really can't see it too clearly (too small) I can't say for sure , but the die could have chipped at that spot . Did you look at Tom's link or many of the threads on Doubled dies in the CT archives ?
This is also a good resource to check if you think you may have a doubled die. I use to to compare against my coins all the time. http://coppercoins.com/advsearch.php
