This deal has ended.
Amazon has continued their buy 3 paperback books and get the 4th free (likely in response to another good book deal: Buy.com's very public "10% off Amazon" price war). Here are the details. Options range from Dr. Suess's Cat in the Hat or Green Eggs & Ham and Harry Potter to thrilers like Da Vinci Code and The Broker to classics like The Fountainhead and Catcher in the Rye. Shipping is free if the 4 books total $25 before the discount and there is no sales tax.