They aren't exactly solid wood
Well, nothing produced in 2025 can match the quality of items made 50-75 years ago. The boards in this unit are not solid pieces of milled lumber. They are more like tongue and groove floor planks covered with a very thin veneer. So, better than particle board, but not 12" wide pieces of wood from one tree. None of it is overtly warped, but still, not fitting together like tongue and groove, it is all butt joints. The back is very thin wood that only spans half the width of the shelving unit, with a janky plastic joiner that goes between the pieces at the seam. Not quite color matched, so it is obvious when you look at the shelf. I would have preferred no joiner, but the backing boards would not be able to be nailed into the unit without a gap if they were flush against each other without the plastic joiner. Assembly wasn't too hard. If you use some muscle and a friend, you can get the joints pretty good — not perfectly flush, but pretty good. You only need a hammer and a Phillips screwdriver, and if you have wood floor, we put down a furniture pad to work on top of to avoid scratching the shelving unit. Overall, if I had infinite time, I could have made a prettier, better quality shelving unit for same amount of money, if bought my own raw lumber, a router, and a table saw. But not having time, these are an adequate but not inspirational solution to our bookshelf needs.