I often like things that look lovely in a set. I confess to having all my Harry Potter books (both British and American editions, because I'm strange like that) lined up on a single shelf, so that one can admire the simple perfection in two sets of seven matching spines (and the snitch that serves a a bookend). I love attention to cover design that puts all of an authors' titles in similar covers so that I can find them easily, and I've been known despite my broke-grad-student status to replace books in order to have a matching set.
Last month, we had a little promo table at the store with trade paperback editions of all of the novels of Emily Giffin, in honor of the film release of Something Borrowed. St. Martin's did a lovely re-release of all four backlist titles and the new paperback of Heart of the Matter in honor of the film release, and they have these wonderfully simple covers in coordinating pastels. As a group, they look pretty stunning. The table was at the top of one of the escalators and next to the booksellers' desk, and so I often wandered past it, straightening the copies and making a point to enter a warehouse order any time we run out of one of the titles.