These are popular books republished and sold at a discounted price by one of these many clubs, the best-known being the Book-of-the-Month Club aka BOMC.
The books are printed in a smaller text format on cheaper paper, using cheaper boards, and the dust-wrapper is thinner or printed on uncoated paper stock. Book club editions can be identified by a small blind-stamped dot, square, or circle at the bottom or near the read board. Most editions do not include the printed price on the jacket flap. An alphanumeric code running vertically up the inner margin of the last page of the text block is also a identifying mark of a BOMC edition book. Only collectible in very rare cases.
See our blog post on Book Club Editions for more.