This is a perfect combo of my two favorite ideas for that space under the stairs leading to the basement. A reading nook AND book storage and shelves . assuming that there is enough space for this there.
27 Ways To Rethink Your Bed: Bed in a Closet. Also check out the Roll-Away Bed under a Platform; Cool way to quickly tidy up a small space, and keep pet hair off the bed! Maybe put a little reading or office space on top on the bed?