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.
Build a Built in Bed With Storage Drawers and Shelves. Now You Can Build ANY Shed In A Weekend Even If You've Zero Woodworking Experience! Your woodworking efforts will be a thoroughly satisfying, enjoyable and ego-boosting experience!