To enable disclosure of information with the aid of turning a book cover inside out by a purchaser in the book cover covering a cover of a book.

The book cover 1 covering the cover 3 of a book 2 has an outside surface 1c of the cover 1 on which a predetermined print showing a bibliography is provided, and an inside surface 1b of the cover 1 on which an arbitrary print unrelated directly to the bibliography is provided, the book cover being mounted on the book 2. The outside and inside of the cover 1 are selectively switched to introduce the book etc. on one hand and disclose the information directly unrelated to this on the other hand.