The following is a text description of a book case I came up with as a shop project for a young lady that I was teaching shop to this summer. I have not included any gross dimensions as they are variable, depending on the customer's needs.
The unit was constructed out of 1x2 straight-grained pine for the sides and 3/4" plywood for the shelves. (Plus hardware.)
The sides of the bookcase look like ladders with the rungs spaced on
6" centers. (They could be spaced on 4" centers for more flexibility in setting shelf spacing.) The rungs have 2" rabbets on each end that are about 1/3" deep and they fit in similar dados in the vertical members. The dados were cut with something akin to a finger jointing jig to keep them consistant in width and spacing.The top and bottom rung have a filler strip added to bring their side surfaces flush with the surface of the vertical members. These filler strips are a little less than 1.5" wide and are aligned with the lower edge of the rung. On the unit we made, the vertical sides extend a few inches below the bottom rung, but they could end flush with the bottom of the bottom rung.
The shelves are cut from the sheet plywood and are fitted with solid wood faces on the front edge. The edges of the shelves are notched to fit through the spaces between the rungs and to extend the shelves past the outside of the rungs by 0.5". A 0.5" x 0.5" cleat is guled and nailed to the bottom of each side of the shelf to capture the sides between the cheeks on the shelves and the cleats against the rungs.
We milled up some mild steel strips about 0.25 x 0.5 x 6" and attached them with #10 wood screws to the outsides of the top and bottom rungs, centered on the filler strips. The steel pieces extend about 1/4 beyond the back of the bookcase. We then made up two cable assemblies with wire loops on each end and turnbuckles in the middle. The loops on the ends of the cables slip over the ends of the steel strips and the turnbuckles are tightened to provide cross bracing so the unit doesn't rack.
When pressure is applied to the side of the uint, it responds by twisting, but if the shelves are a close fit, and they are loaded with books, this is minimized.
This unit is going in a dorm room with concrete walls, so the stell extending out the back isn't an issue. In other environments, it would probably be a good idea to add spacers to the back of the sides to keep the steel from contacting the wall. Also, the unit is going to sit on a flat, concrete floor. If it were being used on a carpeted floor, I would make the front legs about 1/2" longer than the rear legs to clear the tack strip at the edge of the carpet and to bias the unit to lean into the wall.
Thin wood strips could also be glued to outside edges of the shelves to cover the cleats and the exposed plywood edge.
I hope this is reasonably coherent.
Regards, Ed Bailen