I mailed out a WL book on 9/30.  The address was accepted by PBS for printable postage.  On 10/7 the book was received at the post office of the city it was sent to.  Two more scans that day indicate that the book is undeliverable as addressed.  Now the scans are coming back my way and show that on 10/18 it was at the processing center through which my media mail is handled.  I am sure it will be in my mailbox today or tomorrow. 

My question - What is my obligation - do I need to track down the correct address and remail the book?  I have sent a PM to the intended receiver. 

sounds like a possible case of someone moving and forgetting to give PBS a new address. YOU do not owe them a book---you are already out the postage you paid. IF they give you an updated address to send the book to, they must "friend" you another credit to pay for the second mailing, then "accept" the book when delivered for you to get the original credit. (i.e. if you pay to mail twice, you end up getting 2 credits)

What Jeanne said.  If you don't hear back from the requestor, wait for the book to go lost and repost the book.  But, since you used printable postage, at least you received the credit.

This will also happen if the address is unreadable.  In that case, you can take the book, with the full address and old label, to the clerk to remail without paying the postage again.

PBS expects you to contact the member.  You are not expected to remail the book without an added credit if the address was wrong.

And occasionally, the address is perfectly fine, but the Post Office made a mistake.  If the requestor confirms that her address is correct, the Post Office can verify that, and they will send it again "under cover" at no additional charge.

Grreat information.  Thanks. I will just wait now to hear back from the requestor and to get the book back in my mail box. 

What Patty says is absolutely correct.  I've had this happen a couple of times and the postal clerk in charge at my post office has sent it out again free.