Buy an extra bookcase, storage ottoman or take the larger books off and place on your coffee or end table to display.
Make the system easy for the people who actually use it: visible categories, reachable storage and a clear place for each item to return. For younger children, a picture label can work better than a written one.
Measure the real opening and any obstructions before ordering, including hinges, handles, pipes and the room needed to pull something out. The best organizer is the one you can use comfortably every day, not necessarily the one that fills every inch.
Mr. Organized's Answer