We love used bookstores. During our visit to Little Rock, Arkansas this week we patronized “The Bookstore at Library Square.” One of the books we purchased there was published in 1989 and contained the following notice (pictured below) on a loose sheet of paper inside the book: “Your only obligation … is that you agree within a 30-day period to read it and decide whether you want to keep it or not. If you decide to keep the book, you will remit $6.00 to “Books,” 1960 Union National Bank Building, Little Rock, Arkansans 72201. If you elect not to keep the book, then you should pass [it] around in your community as a sharing gift.” Bonus question: is this a valid unilateral contract?