Abstract: In a book binding machine, covers are individually brought into registration with and subsequently adhesively bonded to moving book blocks. The registration is accomplished by first moving the covers along a path parallel to the path of motion of the blocks at a speed which is slower than the blocks and then engaging the covers by an alignment mechanism which at the time of engagement moves in synchronism with the book block as the block overtakes the cover. The alignment mechanism travels about a circular path so as to periodically intercept a moving cover and is resiliently braked when synchronized movement with a book block is established.