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.
For even more storage, consider hanging shelves up high that run the length of the room. You won't loose any floor real estate at all, and you'll gain a lot of storage space. Design Dozen: 12 Clever Space-Saving Solutions for Small Bedrooms