TSM book stacks as functional bookcases. You get three larger ones that go on the floor and two smaller ones that you can put either on the floor or on a surface of your choice - including on the other book stacks, since they all have a slot on top.
One thing I really like about them (I say like I'm impartial or something ), is that even though they're from TSM, they don't shout "MEDIEVAL!" to me. I can easily see them in the study of a contemporary Sim who just likes old books.