Abstract: Each signature feeder along a collating conveyor is selectively actuated under control of coded signals containing mailing information. A dot matrix printer, located between the feeders, is responsive to the coded signals to selectively print custom information within books of signatures. A second dot matrix printer, located after the stitcher and trimmer, prints mailing labels under control of the coded signals. Optical scanners located downstream of the dot matrix printers scan each line of possible printing to detect printing errors. A replacement book feeder, which inserts standard replacement books to fill empty spaces left by rejected books, has a hopper which is automatically refilled by diverting books of signatures from the conveyor. The entire collating line is controlled by a programmable controller and a computer which also reorders or replaces defective books of signatures, compensates for variations in calipered signature thicknesses, and controls other special handling procedures.