Where I live there's a large library-associated biannual book sale where books are (effectively) reverse auctioned over a period of a few weeks. At the end, anything (with a few exceptions, like the Collector's Corner) that isn't sold is disposed of, with a large 18-wheel truck sized dumpster filled with items to be sent for pulping and recycling. The price at the end is $1 for a grocery bag full of books, so things that don't sell truly are perceived as worthless.

https://booksale.org/

Books are information delivery vehicles. We mostly shouldn't care about them any more than we care about a particular set of bits on a disk.

Publishers also pulp large numbers of books themselves. This is a consequence of the Supreme Court's Thor Power Tools ruling, which clarified tax rules in the US so that keeping large inventories of unsold books was less economical.