a "remainder mark?" 

It is generally in black and it looks like a strike / slash mark on the edge of the book.  Generally at the top or bottom edge though I had one on the side edge.

It's put on the books, generally by discount booksellers or when the book is sold at a bargain price in a regular bookstore--it is a mark that alerts clerks in bookstores so that people can't return a book they bought at a bargain price for full price somewhere.

Cheryl

I've seen them in all different colors as well as things like marker dots on the edge of books.

Sometimes it's an 'X' or similar through the ISBN / bar code box on the back of a book. 

Doesn't affect the text and as far as I know does not make the book unpostable