Here’s a clever way to stop shelves from sagging in the middle. Cut plywood panels sized to fit inside some unwanted books. The panels also need to be the height of the space between your shelves. Cut out enough pages to fit the panels’ thickness. Then, build up the sides and front edge of each panel to match the size and shape of the book. Round the edges and corners of the panels to match the books, then paint the panels to match the books or any inconspicuous color and glue them into the books. Stagger the supports a bit so they aren’t lined up right in the center.