My bedroom growing up had the sloping ceiling. When I left home my parents put book shelves on the main wall and each shelf went to the edge so they were different lengths. (We don't have the house any more so I can't take a picture.) The ceiling sloped to about 4 feet, so my dad took the measurement and had a cabinet made to hold his watercolor paper. He had it made to it fit up against the back wall and it was deep enough so you didn't bump your head getting into the drawers. (A friend that collects maps has it now and it is great for holding maps that are stored flat.)
I was watching a show on HGT when they had a sloping ceiling and they made it into a short closet and storage area. It shortened the room but it made a nice storage area.