Remove your books from the bookshelf you already have. Flip over the bookshelf so that the bottom part is facing you. This will allow you to easily add the wheels.
Sand the bottom part of the bookshelf. Wipe away the remaining sand dust. Drill holes in the bottom part of your bookshelf so you can attach the wheels. If you will be using a caster that has an attached shaft, drill one hole for every leg. Ensure that these holes are similar in length to shaft of the caster. If you will use wheels that have a metal plate, drill in the four equivalent holes for every wheel.
Place a tiny amount of wood glue on every hole. For the casters, ram the shaft into the void as far as it will allow. If you are using wheels, place the screws tightly on all four holes.