Toe-Kick
Bookcases shelves should have a toe-kick installed under the bottom shelf just like kitchen cabinets. Toe-kicks are square notches under the front of most base cabinets that allow the front of your shoe to extend slightly under the front of the cabinet. This prevents damage to the front of the cabinet, while allowing you to stand closer to the cabinet or bookcase. It's particularity useful when reaching up to higher shelves. Toe-kick frames can be added at any time.
Remove the Bookcase
Remove everything from the bookcase. Locate all of the screws securing the bookcase to the wall. There are screws along the top penetrating through a solid back rail, through the back along both sides, and there also may be screws penetrating through the middle at random locations. Use a drill/driver to remove all the screws. Place masking tape along both sides of the bookcase along the wall so you can place the bookcase in the exact location from which it was removed. Move the bookcase away from the wall. If it resists, you probably missed some screws. Locate them and remove them. Place the bookcase in a safe location. Ensure it's steady and will not topple over.
Frame
Build a toe-kick base using two-by-four studs or 1 1/4-inch hardwood. Studs are roughly 3 1/2 inches tall, which is perfect for a toe-kick. Measure the length and depth of the bookcase. Subtract four inches from the length. Subtract two inches from the depth. This is the outside dimension of the frame, designed to give the bottom shelf a 2-inch overhang on the front and both sides. Cut the studs to length. Stand the studs on their edges, placing the short pieces between the long pieces. Screw the toe-kick frame together, or use 2-inch staples and a staple gun to assemble it. If you are using hardwood, drill pilot holes for the screws. If you're using studs, you don't need to drill pilot holes. Stain and spray the toe-kick frame to match the bookcase. When the finish is dry, place the toe-kick frame against the wall, evenly spaced between the tape marks.
Mount the Bookcase
Pick up the bookcase. Use an assistant if needed. Place it centered on the toe-kick frame. You should have an equal overhang on the front and both sides. Drill holes through the bottom shelf around the perimeter centered over the top of the frame. Space the holes 12 inches apart. Use a 3/16-inch drill bit. Place a 5/32-inch bit in the drill/driver and drill down through the holes, into the toe-kick frame at least 1 inch. Place 2-inch screws in the holes and drive them in tight to secure the bookcase to the toe-kick frame. Insert the original screws in all of the original holes; all of them should hit the original studs. Drive them in tight to secure the bookcase to the wall.