Pop Quiz

Okay, all you bookish sorts. Big Prize (free book from my Book Mooch listing) to whomever can answer this question.

Why, after the titles of some books at Amazon, are the letters (P.S.) included? Here are some examples:

• One Hundred Years of Solitude (P.S.)
• Daughter of Fortune: A Novel (P.S.)
• The Complete Stories (P.S.)
• Animal, Vegetable, Miracle: A Year of Food Life (P.S.)
  

These are all trade paperbacks, the larger size often with the textured cover, but I can't find any confirmation of this, nor explanation. Any clues?