For this built-out storage unit, use doubled-up plywood, rather than solid boards, to cut costs, add stability, and prevent long shelves from bowing.
The bookcases' sides are equal to 1½-inch-thick planks; make them by gluing and nailing together two ¾-inch plywood sheets. The shelves are 5/4 inch thick, made from ¾-inch and ½-inch plywood. All visible edges are then capped with poplar strips.
To begin, rip sides to the desired depth of the bookcases, cut them to length, and drill for adjustable shelf clips, mapping out the holes with a hardware store jig.
Cut shelves to the same length and depth, but make the ones between the two bookcases longer for a spacious alcove.
Finish by raising both bookcases on a 1x4 platform wrapped with a toekick and shoe molding.