It looks like I'm to fill out the form and send it back to the Mail Recovery Center. It's clear that the sender actually sent the book and while it's not possible to be certain if they packaged it securely, it looks like the help center says that I'm not supposed to mark the book received. Anyone who has experienced this before and can share their two cents would be appreciated as I want to make sure I do the right thing for myself and the sender. Thanks!

From the HELP CENTER:

I received an empty wrapper--no book inside!

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 www.usps.com/postalinspectors (the Postal Inspection Service's website) 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.

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 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.  

It's possible that the post office opened and examined Media Mail and did not re-wrap the book properly!  However, either way, follow the Help Center instructions that Charlie posted.