So you've got a series planned out, and want people to instantly recognize that the thing sitting on the bookshelf there is part of it. What do you do? Give every installment of your series idiosyncratic cover art! This is where cover art is basically the same motif repeated over and over with slight variations within a franchise. Collectors probably love this stuff. Until the scheme is switched midway through publishing a series, anyway. One popular variation is to have the covers of all the installments of a work form a larger picture when they're placed side by side in the correct order.