A book return slot's flap swings only one way, inward, hinged to close on a spring tuned to discourage a hand from following the book too far down the chute behind it. The slot itself sits at hip height in the library's exterior wall, recessed just enough to shed rain, occupying a strange civic office: part mailbox, part chute, part quiet confession booth. Behind the wall, out of public view, a canvas-lined chute drops at an angle set by someone who likely never tested it personally; staff report a soft, muffled thud on the other side, arriving on schedule with the pull of the flap.
Wear collects at the hinge and nowhere else. The paint just above the slot fades faster than the paint below it, worn thin by four decades of hands reaching to push rather than pull. Some slots carry a small brass plate, stamped RETURNS in a font the library itself would no longer choose, and the plate has taken on a shine no polish achieves, only touch.
The slot performs its duty at all hours, staffed by nobody, judged by nobody, and it has never once refused a book on the grounds of lateness, though the system behind it certainly will. Whether this counts as mercy or simply as the limit of the mechanism is not a question the slot has been built to answer. It receives what is offered and lets the wall do the rest.