I was in a thrift store looking at some older books from the late 60's to late 70's... I saw some old Reader's Digest 3 books in 1 books with some nice colors on the cover. There were also some niceold encyclopedias.
Choose however many books you think you might need(depending on the project). Each book is a side of the shelf. They must be hard back and consider the thickness of the books as well.
You will also need the supplies from the https://www.instructables.com/id/Invisible-Book-Shelf/(hot
glue gun, wood scews, "L" brackets, screw driver or drill and a sharp knife.