I love Stepbacks - how could someone not? Not only did you get a nifty cover that greets you, but you can open it for a second cover surprise - extended artwork.
I fell head over heels when I was a teenager because of my whole VC Andrews Gothic drama phase. The old series had amazing family portraits as stepbacks - I don't think they've been done as well since.
There are two forms of stepbacks. The first type are those that have a hole in the cover showing a piece of the stepback beneath, where the face or feature is part of both the front cover and the stepback. The second type of stepback is just an alternate scene, coverart image, where most of the time you can't tell there is a stepback underneath. A few paperbacks indicate a stepback by having the top cover slightly shorter so that the coloring of the stepback shows through.
V.C. Andrews stuff was a classic example of this - a face on the cover merges with the stepback artwork underneath. It's my favorite style of stepback. Uncovering the cover to find a surprise underneath. Reprints tended to have just the cover without the stepback, which was much more plain.
Some Stepbacks have similar artwork on both, just with different backgrounds and the text removed. It's not as common, I guess it's a way to just have a stepback without having to have two pieces of artwork.
Peek Through Stepbacks
Stepbacks that had different art, but pieces of the color of it shine through on a shortened front cover. This way you always know to keep looking. I especially love the Jeanine Frost ones, she has on all her series.