At arm's length on a bookstore table, a mylar jacket and the dust jacket beneath it are easy to mistake for one another, though only one of the two was ever meant to survive more than a single retail season. The paper layer carries the publisher's artwork, printed once, cheaply, sized for a table near a register. The plastic layer arrives second, applied by a bindery worker who folds the excess mylar behind the jacket's flaps and tapes it down, a process that takes under two minutes once learned and considerably longer before that.
The material announces itself acoustically before anything else. A jacketed book pulled from a tight shelf produces a specific crinkle, brighter and higher than paper, audible from an aisle away, and patrons learn without being told which returns cart holds the newer acquisitions purely by ear. The sound softens with age as the mylar develops a network of fine creases along the spine fold, the plastic equivalent of a wrinkle, though it never goes fully silent the way cloth eventually does.
Underneath, the paper jacket keeps its color for years longer than an unprotected copy shelved in direct light, the mylar filtering nothing in particular but somehow slowing every kind of decay it stands between. It yellows faintly itself, eventually, at the corners first, and no one has quite determined whether it protects the jacket by absorbing damage meant for the paper or simply by being harder for a careless hand to fold or tear. Either way, the two layers are catalogued and shelved as a single object now, the seam between them invisible unless you go looking for it with a thumbnail at the corner.