Any idea what the literature term of appearance is?

I’m not sure I understood your question correctly. Are you asking what “appearance (external)” means as a point of comparison between two books? Is that phrase one of your options on an instruction sheet?

If so, I would try first to remember what the teacher said about the options and points of comparison. It’s the teacher’s interpretation that counts for the most in your assignment.

Given no other clues, I would say yes, it means the outward appearance of one or more characters. (I don’t know how “appearance” can be anything other than external.) Possibly it means the setting.

But it is hard to think of this as a dimension for comparison between two books. Say one has a main character who is an old man and the other has a main character who is a young woman. They look different. So what? Their looks don’t offer anything to discuss or analyze in relation to each other. People look different. Period. I would certainly choose another element if I could.

Do you mean character traits?

Setting? Landscape? Description of characters? Color and size of physical book? You probably have to ask your teacher for clarification.

Do you mean form? i.e. the structural elements of the books?

I think the external appearance a character doesn’t have a special term. I would just use ‘physical description’

If you are describing how the character is percieved by others (vs. an internal self image) you can use ‘public persona’.