Remaindered books or remainders are printed books that are no longer selling well and whose remaining unsold copies are liquidated by the publisher at greatly reduced prices.

Copies of remaindered books may be marked by the publisher, distributor, or bookseller to prevent them from being returned. "Remainder marks" have varied over the years, but today most remainders are marked with a stroke with a felt-tipped marker across the top or bottom of the book's pages, near the spine.

https://en.m.wikipedia.org/wiki/Remaindered_book