http://varietyvista.com/What Are Die Varieties.htm Looks interesting...does it describe the different varieties of 1955 D/S nickels?
I think you will like it. The e-book is very well done. The photos are excellent. The author (Dr. James Wiles) gave me my copy last summer because I had taken his mini-seminar (The Modern Minting Process and Errors and Varieties) the previous summer.