Booklets are where new prepress people get humbled. The customer sends 44 pages in reading order; the press needs those pages shuffled, paired, rotated, and nudged — and every step has a failure mode that only shows up after folding.
Reader order → printer spreads
A saddle-stitch book is folded sheets nested inside each other, so each sheet carries four reader pages in a non-obvious pairing: the outermost sheet holds pages 44 + 1 on one side and 2 + 43 on the other. The general rule for a book of N pages: spread k pairs page (N−k+1) with page k. Do it by hand once to respect it; automate it forever after.
Multiples of four, no exceptions
Physics: each folded sheet = four pages. A 42-page file becomes a 44-page book with two blanks — decide where those blanks land (usually before the back cover) rather than letting them surprise the customer.
Spine creep
Nest twenty sheets and the inner ones push outward at the fold — on 80# text (≈0.004" caliper) an 80-page book shifts its center pages roughly a sixteenth of an inch. Trim the face flush and inner page numbers walk toward the edge, or off it. Fix: shift each sheet's content toward the spine progressively, proportional to its depth in the stack. It's caliper × sheet-depth — arithmetic, not art — but skipped constantly.
Digital vs offset plating
Same booklet, two right answers. Digital: step one signature per sheet — the press collates sets, you make one cut, and the books fall together in order. Offset: gang all signatures on one image for a long identical run, then cut and gather the piles. Shops get burned using the offset style on digital work: it looks efficient and then someone hand-marries piles all afternoon.