The Monopoly Shoe
The shoe was the piece Susie uses whenever her family played monopoly. So when Jack had to explain to 4-year old Buckley that Susie is dead, he used the shoe to symbolize Susie's death.
Jack said to Buckley, "Right, we'll make your friend Nate the hat and the board is the world. Now if I were to tell you that when I rolled the dice, one of the pieces will be taken away, what would that mean?" (p.69). In a way, in Buckley's mind, the shoe represented his sister. He cherished this shoe like he cherished his sister by keeping the shoe as a remembrance of her. Not surprisingly, Buckley was very upset when the shoe got lost. Losing the shoe meant losing Susie once again.