The Diet Book is a key item that appears in Super Paper Mario.
This book belongs to Hagra of The Underwhere. Hagra lent this book to a D-Man awhile ago and wants it back but is too old herself to get it. When the heroes agree to help her out, Hagra warps them to the D-Man that has the Diet Book. The D-Man gives up the book saying it was just a fad anyway. Giving the Diet Book back to Hagra results in her giving the heroes the Door Key which is needed to proceed.