Some of the teachers in our first grade classrooms use book bags from Really Good Stuff. On the classroom white board, there is a metal rail that the teachers drill holes big enough for a "s" hook to go into. The hooks hold the bags with books in them really well, and they don't take up much useable space at all. I know this may be hard to get a mental image of, so I'll try to remember to take a picture to add later.
Lakeshore Teacher Supply has a lightweight frame that holds 12 narrow book boxes (which they also sell).