Here's the rules:

If this is a book you requested, you should not mark the book received, since you did not get the book.  You should contact the sender to let him or her know what happened.  

• The sender should call 800-ASK-USPS or go to http://postalinspectors.uspis.gov/forms/MLNtRcvd.aspx  and input the information online there. This will help USPS try to recover the book.

  • The form is titled "mail Theft"; this does not mean that you can use it only if you think the book was stolen.  "Received without contents" is an option on the page.
  • The choice to make under "Class" on the form if Media Mail was used is "Parcel Post".

• If you think that the Wrapping method may have contributed to the loss of the book, please do give (gentle) feedback to the sender; you can refer to All Help documents about bookwrapping if you like. 

• If you have marked the book received by mistake (or if the system marked the book received for you while you were absent from PBS), you can also discuss this with the sender, who got credit when that happened. See How do I contact another member? for instructions on how to contact a sender from the transaction in your Transaction Archive.

If you have not marked the book received yet, do not do so.  You will continue to get the usual auto-emails from PBS asking if the book has arrived; open and read them, and follow any instructions within. 

If the book hasn't arrived by the 26th day (the 35th day if the sender or requestor is outside the contiguous US) after it was marked mailed, then PBS will declare it lost in the Mail and you will get your credit back.  If the book does arrive after that, you should mark it received from your Transaction Archive.  

If the sender agrees to cancelling the request early (not waiting the full period for the book to be declared "lost"), then she or he can contact us and ask us to cancel the transaction for the book.  (A cancellation in this case will pass your request to a new sender if there is another copy of the book available in the system, or will put the book on your Wish List if there is not another available copy.  If the book was on your Wish LIst before, our cancellation will put you back on the Wish List for this book, in your previous place "in line".)

Thanks for the info.