• 2 sides at 48"
• 4 shelves at 34 1/2" (top, bottom, two middle)

Square ends matter here — use a guide with the circular saw or a stop block on the miter saw so all four shelves are identical.

Drill three pocket holes in each end of every shelf, on the underside so they're hidden. Set the jig for 3/4" material.

Clamp the two sides together, edges flush, and mark both at once: bottom shelf at 3", middle shelves at 18" and 33", top flush at 48". Marking them together is what makes the shelves come out level.

Screw the bottom and top shelves between the sides first — that makes a rigid frame. Then add the middle shelves at your marks. A helper (or a couple of clamps) to hold shelves on their lines makes this painless.

Measure the diagonals, rack the case gently until they match, then glue and brad-nail the plywood back on. The back is what keeps a bookcase square for the next thirty years, so get the diagonals right before nailing.

Break all the edges, sand to 180, and apply two coats of poly with a light scuff between. If it will live on carpet, add small levelers; if children live with you, add an anti-tip strap to the wall.