Abstract: A cover breaker mechanism which functions to shape a cover to the back of a book which is moving in one direction. The cover breaker mechanism includes a bottom plate for engaging the bottom portion of the cover which forms the back of the book and a pair of side bars for engaging portions of the cover which lie adjacent the back of the book on opposite sides of the book. The bottom plate and side bars are carried on a support which moves from a start position in a first path which causes the bottom plate and side bars to act on the cover, and in a second path spaced from the first path for returning the bottom plate and side bars to the start position. A means is provided for moving the support which includes a pair of members which are supported on respective eccentrics.