Attach this to the top of your Billy bookcase and get space for an extra shelf (or two if they are really short shelves). Simple to install. It is intended for a 31 inch wide Billy (any height), and when added, makes it 13 inches taller. If added to the tallest Billy, you get a 7.5 foot bookcase. Not bad.
Here's the product on Ikea's site: https://www.ikea.com/us/en/p/billy-height-extension-unit-white-80525278/?msockid=143725a6a9e663b417b23087a885626b
In good condition, a few small dents/markings (see photos) that aren't noticeable when it is installed.
Pick up two blocks from Union Square. Include available pick up times in your response (must be weekday after 6 pm or daytime on the weekend). Thanks.