Decorating a Box Like a Book?

Measure the height and combined length of three sides, not measuring the back side. Find a ribbon or cloth piece that looks like it could pass for the closed pages of a book. Measure this ribbon or material to fit around the three sides and glue in place.
Measure the top (hinged lid), back side, and bottom of your box. Find material to fit that area that looks like a book cover and glue in place. Decorate further as desired.