Japanese people call it a “Book Cover,” which keeps a book clean and protects from other people to see what you’re reading. We have about 4 main book sizes in novels, so we need at least 4 “Book Covers.” However, only you need is one if the size can be “Adjustable.”
You can make the “Adjustable Origami Book Jacket” with 4 sheets of copy paper (both A4 and Letter sizes work). You just need to fold them like Origami and put them together like a slide door. No measurement, cutting nor pasting required.
I am interested if there is other country use “Book Cover” like we do. But even if you haven’t used it, try this, because It just fun to make.