ooh this is great for unfinished basement stairs (book-staircase). I love the idea of book shelves in between the stairs. This would be especially helpful for a small home and a book lover that needed more shelves.
Staircases take up a lot of space, so why not use them for storage as well. Imagine each step is a box with a hidden drawer in it still allowing for additional storage or home office space beneath the staircase.