I like the books sitting upright just as well as I like them stacked. Unless you're implementing this idea because you really need more storage space for books, it's a nice way to display a favorite item or two - even to use for no other purpose than to create "artwork" for an empty wall. Probably does need a book or two no matter what its purpose, to maintain the "theme" inspired by the "book" shelf. ;-)

I have to comment again on Polymathism's staggered zipper design, too; it's also a very attractive example of mixing it up a bit, by adding a single rather dramatic piece on the uppermost shelf. I love where both of you went with the idea almost as much as I love the instructable!