Abstract: A book sewing machine has an inserter for plates for indicating the planes of separation between the books. Each plate is disposed in the separation plane in correspondence with the top of the adjacent face of the contiguous books on that side of the books facing the loader. The machine also has a gluer for applying a layer of glue to the backs of the books which advance towards the discharge zone. Close to the latter is a separator constituted by a cutting blade which traverses the pile of books so that it cuts the layer of glue on the backs and simultaneously cuts the thread connecting the last book to the next book, thus effecting the separation. An operating cycle of the separator device is started each time a control circuit is given an enabling signal by a detecting device for detecting the passage of a plate. The position of the detection device preferably coincides with that of the cutting blade which may be a separate element or which may be part of the plate.