Summary

To make a book shelf you need to print two of these brackets, a wood board and screws.
The board can be bought from a home improvement store. Thickness: 1" (actual width is 3/4") and Width: 8" (actual width 7-1/4"). (Note: in the US stores the dimensions correspond to the sizes before the finishing process.) Length is as needed for a specific case.
Two 1-1/2" #8 wood screws fix the bracket to the board. Another two #8 screws (and possibly anchors) are needed to attach the brackets to the wall.
The assembly can be seen here: https://youtu.be/I67959l5sLI