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.
The whole window will have a one foot wide window sill. I will have to show you the place to get a good idea of things. Basil planted in teacups for the kitchen, cheap and pretty teacups planted with fresh herbs on the window sill.