If by "design" you mean aesthetic principles, I don't know. You can get engineering spec's and requirements for terraces and retaining walls, etc. from a number of websites and governmental agencies. Your local municipality or state may have spec's and requirements posted on a website or available for public reading. I remember seeing some stuff on the EPA's website amongst others.

Thank you Bob64. I am indeed looking for "aesthetic principles". I should have been clearer about that.

Have you looked at native/ornamental grass? I think they would look beautiful from below and above. They have deep roots that aide in keeping soil in place.
Lovelycherry

I think this is one of those things you may have to learn as you go.
I don't think this has been looked at by landscape designers.

I am doing a steep eroding hillside and my first thought is to keep the soil in place rather then how it will look from below.
I do have an audience for my hillside (what I really mean is nasty eroding weed infested sandy slope), the train goes by slowly as it pulls out of the station so all the riders will be looking at it.
So I now have a new challenge how it will look to the public that passes it everyday.
Lovelycherry

I think your aesthetic possibilities will be narrowed down by the engineering needs of the slope. For instance, if you have a slope that needs terraces then your aesthetic possibilities will be different than if terraces are not needed. If you have a hill that can support large trees your possibilities are different than a slope whose angle of repose does not permit large trees. Of course, there are still lots of choices to be made even so. For instance, with terraces there are some choices to be made within the range of viable materials based on your taste and budget (stone, wood, angular, rounded, etc.). Some people like revealing the "bones" of the hill. Others like to cover up the hill. Some people like bare rock, others like moss covered rock. The choices go on and on.

why would the design principals be any different for a hill?

Your decisions about how to reach the design would be different in some cases, but the principals would be the same.