Measure back opening (the plywood sits between the protruding outside edges of the sides that were formed from the rabbets that were previously milled, and fully overlaps the backs of the carcass top and bottom panels). Cut 1/4″ plywood to fit snugly. Even though I have a large cabinet saw equipped with ample outfeed support, I like to make the cross-cut first using a circular saw for a safer, more controlled operation, and then rip to width on a table saw. Be careful when cutting 1/4″ plywood on a table saw, as it can flex, allowing the material to rise above the blade which can cause a dangerous kickback. Use a slow feed rate, apply steady downward pressure when needed, and be sure to use your blade guard for this operation.
Then, beginning several decades ago, shop classes began to be removed from secondary curriculums. With the decrease in educational funding, and increasing emphasis on standardized testing, schools began to cull electives, institute stricter graduation requirements, and focus more on college prep academics and the subjects necessary for passing state exams. There wasn’t money or time to maintain tools and sawdust-filled workshops, and one by one school districts dropped their shop classes, figuring that students who wished to pursue trade skills could do so later at a vocational college.
The knotty pine bookcase parts are all standard dimensional lumber that you can find at any home center. We joined the shelves and legs with biscuits. If you don’t own a biscuit joiner but still want to build this project, you can simply nail or screw the parts together and fill the holes. We used a table saw to cut the 1-1/2-in. square legs from 2x4s and a router with a 45-degree chamfer bit to bevel the edge of the top. If you don’t have a table saw or router, you can just use stock 2x2s for the legs and leave the edge of the top square.
There are several ways to complete this task, including pocket screws, brads or biscuits. I have used all of these methods, but my preference is biscuits, as they provide adequate strength, excellent alignment, and ease. I generally just place one biscuit in the center of each rail and stile, which provides alignment assistance while not being overly constraining when manual adjustments need to be made to achieve desired placement. After applying glue and biscuits, use lots of clamps to create a gap-free bond between the face frame and carcass.
Use 1x12 pine lumber for a rustic appearance. Nothing looks like pine with its large knots and swirls. And because you're using solid lumber, you can sand the edges smooth for a finished appearance. Add a 1/4-inch fir plywood back for strength and stability. Note that "1x12" is a nominal size. The actual size will be ¾-inch thick and 11 ¼-inches wide.
Another fine example of remodelling furniture and re-using what already exists in the house.  The bookcase involves using your old kitchen cabinets, and building new shelves on top of them. The final product can also be used as an entertainment unit, housing a television, books and things of your interest along with ample storage to keep other utility objects.
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