Here is a photo of a guy's room I designed that may help with the bookcase idea. This room features a bed but can easily be transformed by installing a window seat, desk, or display area. This was all custom shelving (paint grade) using the window as the pivotal lines for designing the bookcases. In later years when this teenager left the nest, the bed was removed and replaced with a sofa.

The pleated shade on the window tucked under the wood valance so that when it was up it could not be seen. The TV is on the perpendicular (L) wall. Be careful putting a TV on an opposite wall because of sun glare. There may be times during the day when the sun may interfere with the picture.

Notice the use of different paint colors on the walls. A little hard to discern in the photo, but there are 2 shades of the same hue. The lighter on the slopes and the darker on the straight walls. When you are working with a slope such as this, treat the slopes like they are walls. You should also include the same color onto the flat portion of the ceiling where the slope stops. Treat it like a wall also to make a cocoon of color. Ceilings are really your 5th wall. If your room is colorful, why have your ceiling white? Great space - Good luck.