The symbol, usually red or black, is a remainder mark used by the publisher to identify books that have been discontinued and removed from its active catalog.
Publishers sometimes offer to sell discontinued titles to bookstores at a discount. Since stores can often send unsold titles back to the publisher for credit, the publisher knows to credit the store a discounted amount for returned books with reminder marks or not to accept the books at all.
A remainder mark is considered a fault and somewhat reduces the book's value. How much it reduces the value is debatable, since roughly 90% of a collectable title's value is in the dust jacket.
