This invention provides a system and method for aligning, feeding, trimming, slitting, rotating, cross-slitting and stacking sheets, each containing one or more discrete page images thereon that allows for greater automation of the overall process so that reduced or no manual intervention is required to generate completed book stacks or “blocks” from a stream or stack of printed sheets. Sheets are fed downstream on a feed surface, trimmed at a first, upstream trimming station to remove margin edges and optionally separate the sheets relative to the discrete page images. The sheets are then rotated 90 degrees and fed to a second, downstream trimming station that trims the right-angle edges and optionally separates the sheets into a final group of full-bleed pages, removing margins and gutter strips. The sheets are feed to a stacking assembly to be tacked in page order and any rejected, defective sheets or stacks are removed from the order. Trimming stations can be automatically adjusted to accommodate different numbers and sizes of pages in sheets. The stacking assembly can include a right-angle merge assembly that can include a plurality of decks, served by divert gates to handle a large number of separated pages to-be-stacked.