Folded cover sheets are conveyed seriatim from a first magazine into each of several parallel channels during each of a series of successive intervals. Such sheets are opened up on entry into the channels and are transported past one or more locations where they receive inserts which are supplied from one or more additional magazines, again in such a way that each additional magazine admits an insert into each channel during each of the successive intervals. The thus obtained groups of sheets are gathered into larger groups which are assembled into a continuous stream of partly overlapping groups.