Book bindery and trimming apparatus which is directly connectable to the output of a digital printer enable substantially automatic book production. The task unit includes instructions for executing an application task. Wherein, the common line and the branches thereof, and the pixel electrode constitute the storage capacitor. Its outer surface is concave with respect to the outer surface of the peripheral auxiliary air-chamber, that is, the surface of the peripheral auxiliary air-chamber extends beyond the surface of the bottom sheet of the lower main air-chamber.