"See these built-ins? (They) hired a local handyman to build them to fit between those two windows and provide a ton of kid-friendly storage. So smart. I especially liked that they chose doors that are flush with the frame (it had a really clean look)." - also like that they're not too deep, they just fit with the room!......House Crashing: Cozy & Full Of Character | Young House Love
built ins around window seat? use standard cabinets for the base of the bookcase, build shelves from 5/8" plywood. The long shelf about seating should be anchored to wall and ceiling as well as the side bookcases.