The sender should call 800-ASK-USPS and give the information (or go to http://postalinspectors.uspis.gov/forms/MLNtRcvd.aspx if that link works - it doesn't always work, since USPS changes things - and input the information online there). This will help USPS try to recover the book.
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.
Related Links:
I received a book that was 'lost in the mail'
How do I contact another member?
I accidentally marked a book received!