Lay a 12-foot wooden ladder flat on your work surface and cut it into two sections that are 6 feet each. Count the number of rungs on one ladder. This is the number of boards you will need.
Visit a lumber yard and purchase wood boards that are around 1 inch thick, 11 inches wide and 3 feet long.
Place your ladders on their sides two feet and 10 inches apart. Adjust them so they are parallel and even. Place a board on to the top two rungs.
Adjust the board so it is even between the two ladder pieces and drill the ends of the board into the rungs of the ladder using a handheld drill and two screws per rung. Repeat with each subsequent board.
Stand the bookcase upright and attach L-shaped brackets to the top and bottom rails of the ladder. Run a stud finder against the wall, marking the areas were you find studs with a piece of chalk. Drill the L-brackets into the wall where the studs are.