The final thing we are adding to our son's room is a two-foot wide built in bookshelf, where his old closet used to be. It's 10" deep, which allows for an 18" deep linen closet backing up to it.
Here is the bookshelf unit before it was installed. This is a very simple design: 3/4" plywood sides and shelves; 3/8" bead board plywood for the back; MDF face frame pieces. I sanded, primed and painted all the pieces before assembly, saving a lot of effort.
Here is the unit installed in the wall, with side trim applied. It is shimmed solidly in place, with finish nails holding it.
This bookshelf matches the other ones we installed in our basement, to give continuity throughout the house.