An adjustable book cover (10) consisting of a unitary hexagonal sheet of stretchable fabric (16) that is preferably coated with an elastomeric polyurethane resin (15). Interstices (19) of the fabric (16) are filled with at least a single layer of polyurethane resin (15), creating an impermeable waterproof barrier while maintaining the elastic functionality of the book cover (10). The hexagonal sheet has flap portions defined by fold lines (44,46,48) that delineate a laterally formed rectangle within the fabric polygon. Extended edges of the upper and lower shaped flap portions are each bisected by a slot (41) of predetermined depth, thus forming an access aperture (21) for the covers of a book when selected flap portions of the hexagon are secured together.