I designed this bookcase using the Golden Mean ratio. The narrowest part of the side frames (where the middle shelf is located) is 0.6 the height of the bookcase.
The dyed maple frames are doweled except for the top of the center stile, which has two pocket screws. (The center stile and lower front stretcher are not the same thickness as the other frame parts, giving the project a nice shadow line.)
The frames are attached to the carcase with biscuits along the top and bottom, but splines and loose tenons align and strengthen the joinery. The 10×48 black marble top just sits on top of the carcase, nesting inside the frames.
I built this prototype without the marble top and used plywood for the carcase. Also, the frames are only 3/4 inches thick instead of the design size of 1-1/8 inches.