This is the perfect place to put a pull out bed. I always look at those beds and think they're impractical because it's too hard to leave all of the open floor space needed for one but I'd happily do it for this library/reading space
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.
Idea for tiny house if you have stairs to loft. The depth of space under a staircase can be sufficient for a built-in entertainment center that houses a flat-screen TV and a bookcase to boot, along with drawers for remotes, movies, and game accessories.