@Mr ATC No glue. I am not comfortable using wood glue yet. Glue also would have slowed down construction. Step 1) Cut pieces to length then width. The main supports are pallet risers (2x4s) cut down to ~1/2 width. Step 2) Lay 2 risers parallel. Nail cross members to it flush. (Square is largely established by the flushness and the uniformity of lengths.) Narrow cross member at bottom supports the shelf. Top member encourages square & enables vertical lifting. Middle member keeps books from falling out the end. (x2) Step 3) Lay both ends face down. Nail back members flush. These members tie the ends together and keep the books from falling out the back. Step 4) Install shelf. Lay the wide shelf boards atop the narrow bottom board. This will help make the front more rigid. Install the front board first. Nail in place. (Optional: Install narrow middle bar under the shelf to reduce flex in the shelf if using thin wood. 1/2" thick shelf boards have very little flex in this short span.) Step 5) Add top boards. Again, these make the front stiff and help transfer weight from above onto the thicker verticals. Front board first. Nail into place. It's easier to nail into the verticals, but w care you can also nail into the top cross member on the ends. Done?

@Mr ATC No, it's all brad nails shot w my gun.