Listen to Otis on patent stuff.
When I read knock down, I thought you meant something that would just fit together. What you are meaning is what I would call take-apart.
A long time back I made a batch of knock down book shelves. They are about 18" wide, about the same deep, and about a foot tall, and 4 pieces. The bottom piece is slotted about 1" or so on each side, halfway, to accept the side pieces. The side pieces are slotted an 1" or so from back to front at bottom, and slide in the bottom, and is also slotted at the back, from top to middle, to accept the back. The back is slotted to fit the two end pieces. All 1/2" plywood, very sturdy, don't know how much weight they will hold, but a lot. The sides are angled just a bit, so you can stack them. Anything over 4 levels tho they tend to start tilting. You can cut a piece that will fit in the bottom of the piece on the floor. I also cut pieces that go from side of one to the side of another, making a shelf. One I made a piece with 'guides' to fit on the top shelf, making a shelf or top. I really don't know if they would sell, probably not, but are remarkably handy, and very quick and easy to put together, or take apart. What I especially like is, no fasteners. I guess you would call them puzzle shelves. I have my hallway lined with them, filled with books and magazines, one with a top by my front room seat, and a number in the shop. Think I still have the masters too, if I ever want more. Sounds simple to make, and singles are very simple. However, to stack them you need a slight angle on the sides, and that is tricky, you will need to make a few before you get that right.