Abstract: A stack of sheets of sheet material is stapled along an axis and fed to a position from which it is folded along the axis by a blade to form a booklet having a curved spine. The blade also inserts the booklet into a position between clamping jaws and determines its final position therebetween. Rotatable guide rollers guide the sheets into the gap and are fitted with one-way clutches to resist upward movement of the folded sheets upon retraction of the blade from the gap. In a first mode of operation, the curved spine of the booklet protrudes a short distance beyond the jaws. The spine is then flattened by a forming roller which passes along the spine. In a second mode of operation, the spinal portion lies between the jaws and the curved end surface is flattened by the clamping force alone. The first mode is suited to booklets having or more sheets; the second to booklets having fewer than sheets or having loop staples and any number of sheets.