BOOK

PROBLEM TO BE SOLVED: To provide a book having a paper surface spreading larger than the book when opened, automatically developing paper pieces in the case of opening the book and automatically containing the pieces when closed. SOLUTION: In the book comprising long lateral rectangular mounting sheet pieces 1, 2 and enlarging forming piece 4 adhered to opening/closing inner parts of the pieces 1, 2, the piece 4 is a substantially square shape having sides longer than a longitudinal short piece of the long lateral rectangular shape and shorter than a long lateral piece. In this case, the piece 4 has an angle folding line on its connecting part and valley folding lines 8 toward points 7 near the mounting sheet side near four corners from a central point on a diagonal line and has an angle folding line 9 toward four corners from the points 7 and a valley folding line toward the side of the enlarging forming part nearest the point 7, and right and left parts surrounded by the valley folding lines of the piece are respectively adhered to the piece 1 and the piece 2.