Method and apparatus for assembling sheets of printing media for booklets. In one aspect the sheets are folded, sheet-by-sheet, and in another aspect the sheets are collected, sheet-by-sheet, and registered on a fold in each sheet. In still another aspect printed sheets are loaded, sheet-by-sheet, into the apparatus. Each sheet is trimmed to a pre-determined width depending on the position of the sheet in the booklet being assembled. The sheets are thereafter folded, sheet-by-sheet, and collected into a stack. The method and apparatus have particular application in finishing duplex printed sheets of paper into saddle-stitched booklets.