If you want to buy these, they cost 120 USD for four (Kartongroup). Too expensive, I thougt, so I made them myself.

Step 1: What You Need

You need:

- a big piece of solid cardboard
- pen
- cutter
- wood glue
- rulers
- scissors

Step 2: Template

I made this template at first. The measurements of the real thing are on the template: height on the second picture, width on the third picture. This template has only three parts, but you obviously need four parts in one piece to make the bookshelf. The cardboard I had for the template was too short for that.
Fold the template so you can see if there are any flaws.

Step 3: Drawing and Cutting

It takes a while to cut through thick cardboard. Good luck!

Step 4: Folding

To be able to fold some parts, you need to cut through half of some lines. Be carefull to not cut them all the way through! Fold the whole thing before you start gluing.

You can see I left a bit of extra cardboard on the left end side. I wanted to use that to glue the ends together, but that doesn't work, so you need to cut that off.

Step 5: Gluing

I used wood glue and it holds very well. I put the glued parts between a workbench to dry for a day, piece by piece.

Step 6: Finishing

To close, I glued some cardboard on the inside of the ends and let it dry. Then I put glue on the other sides and shoved them into the other ends to make the square.
When you glue the last part, be sure to put the whole thing in the wright position to let the glue dry, so your bookshelf doesn't end up awry.
Finished!

Make some holes to hang it on the wall, or make several of these and put them on top of each other. You can also make several pieces with different sizes, you can paint them...
Let me know if you make this!