Ikea is great because you could buy one piece now and add more Billy shelves later, you don't have to buy all the pieces that make up that "set" of bookcases now if it isn't too far of a drive to Ikea for you.
I know what you linked to just shows one article number, but when you get to the store and try to look up where to find it, you are going to get a list of all the different boxes you need to make up that set.