My kids used to have their books covered.
I normally bought a roll of brown shipping paper & used that (since most of our stores use plastic bags.
Set your book in the middle of the brown paper, make sure that there are about 2" of paper around the book (could be more - I used to like 4").
Crease the paper at each of the four ends of the open book, centering the book in the middle, so that there is a crease at the top & bottom, and at the ends of the book cover.
Fold the paper at the top & bottom creases.
Put the book in the middle, and bend the left or right crease of paper, then slip the book in.
Fold the paper over the book, & slip the other end of the book into the paper cover.