Apologies, all--didn't realize I hadn't already explained it. It's a triangle enclosing a large S, printed on covers of books that are considered "strippable"--that is, those for which booksellers may simply tear off the front cover, properly dispose of the rest of the book, and return only the cover to the publisher for credit on the unsold book. It's not used universally yet, but you'll see it if you look for it.
And now you know. Though I think an edible undies story might have been more exciting. :-)