There are two ways I would suggest doing series or trilogy covers based on the content in the books. For single POV books: I’d recommend using the same model or character design for each cover but using different colors, situations or background elements for each. For example, the covers I created for LM Preston’s The Pack series, featured this type of design. If instead you are using symbols or other non-human design element such as Veronica Roth’s Divergent series, all the covers should use similarly important symbols. Color isolation, like that used in the Twilight Saga covers, is another good example.