A saddle stitcher apparatus (90) for forming completely finished reproduction sets of a series of reproductions produced on sheets, respectively by a reproduction apparatus (12), is one of a group of modular devices (30, 50, 70, 90, 150) of a modular finisher apparatus (10). The saddle stitcher apparatus (90) comprises a support frame (118) including means (100) for mating with means for detachably accepting the saddle stitcher with a housing (32) of the modular finisher apparatus (10). Means associated with the support frame (118) define a sheet feed path (P₃) communicating with reproduction sheet receiving means (E). The saddle stitcher apparatus (90) comprises means (96) for stapling the registered sheets at the mid-line thereof, means (94) for folding a stapled set along the mid-line thereof, and an output hopper (98) for collecting folded stapled reproduction sets.