Abstract: The method and apparatus for printing the side edges of sheet stacks, such as books, catalogs, and the like, wherein after the stack side edge has been trimmed and cut the stack sheets are maintained in relationship under slight compression within a belt conveyor using upper and lower belts. As the retained stack is translated through the printer a sensor senses the location of the stack leading edge initiating a timing circuit which controls a rotary printing head engageable with the stack side edge to print indicia upon the side edge at the desired location. The movement of the stack rotates the printing head and a dense high quality image is produced. In an embodiment of the invention, a plurality of printing heads are mounted upon the conveyor apparatus to print the stack edge, sequentially, with a plurality of colored ink.