I've been in second hand bookshops that sprinkled their copies of The Da Vinci Code around the shelves so it wasn't so obvious how many they had.

(When I was a kid, Dennis Wheatley paperbacks were everywhere. Now I never see them. They were pretty cheap paper, maybe they literally fell apart).

A lot of older paperbacks seemed to use a lower grade of woodchip fibre and truly awful glue. Admittedly I live in the subtropics which is very unkind to books but it's my airport thrillers, cheap scifi and old penguin and pelican books.