Remove the backing from a three-shelf bookcase. Measure the backing to get the measurements for a hardwood replacement. The lumber yard will cut the hardwood to size for free or a nominal cost.
Sand the bookcase and hardwood backing with 80-grit, and then 200-grit sandpaper. Remove any sawdust with a damp cloth. Paint the pieces with an oil-based primer and allow to dry, then apply two thin coats of semi-gloss paint, allowing to dry in between.
Drill guide holes in the hardwood backing with a 1/4-inch drill bit when the paint is completely dry. Nail the backing to the bookcase with 1/2-inch wood nails.
Measure the length and width of the top of the bookcase. Have a piece of 1/2-inch butcher block cut so that, when installed, it will extend 4 inches beyond the back of the bookcase. For instance, if the top of the bookcase is 12 inches by 36 inches, the butcher block would be 16 inches by 36 inches.
Apply epoxy glue to the top of the bookcase. Set the butcher block on the glue so that the front edge aligns with the open side of the shelves, and the excess extends past the closed side of the shelves. Set garden bricks or some other heavy objects on top and allow the glue to dry.
Turn the whole unit upside down when the glue is dry. Use a drill to screw lag bolts through the top of the bookcase and into the butcher block. To determine the size of the bolts, measure the thickness of the bookcase top and add 1/4 inch. So, if the bookcase top is 1/2 inch, use 3/4-inch lag bolts. Screw in a bolt at each corner, and half way between each corner on the long sides of the unit.
Drill guide holes into the four corners of the bookcase, using a 1/4-inch drill bit. Screw a 4-inch-tall furniture leg into each guide hole. This will add height to your bar. If the bookcase is tall enough as is, the legs are not necessary. Turn the unit right-side up. Place two bar stools at the back of the case, where the excess butcher block extends.