The signatures in this mass market paperback (Weapons of Math Destruction, Cathy O’Neil, a great read, BTW) continue to group together, even though the book is perfect bound, and the spinefolds are cut.
Paper memory. Not enough pressing or pressure during or after folding. We could reconstruct the imposition based on this visual evidence, even though there are no conjugate leaves or other signature markings.
Whatever glue was used on the spine (PUR?) grips the newsprint-like leaves solidly, with no throw-up and lots of drape, making the book easy to hold and read. I’m embarrassed to admit this, but the tactile qualities are very satisfying in this book, at least in the short term, better than many high-end hand bound books.