Ideas for Deep, Tall "Book Shelf"

So I have moved into a Hostel and the "Shelf" (not sure of the exact term, but it's essentially a shelf built into the wall with concrete and stone slabs) which I have earmarked for my books in not very conductive. Its too Deep ( 1'6") and tall (1'6") and the width is 3'5".. If I place the books vertically against the back wall, there's too much unused space above and in front. Placing them horizontally makes retrieval difficult. I'm considering using bricks and plywood to create two 'floors' to take advantage of the extra height, but the depth is still underutilized. Any ideas on how to maximize the storage capacity for my books in this space?

PS: At this point practicality (and cost) is more of a concern to me than aesthetics.