I sent out a book on Nov. 21st and I got it back in the mail today.  It was marked

Return to sender - attempted - not known - unable to forward.

I sent the member a PM with this information.  Is there something else I should be doing?  I noticed this is a new member (Nov. 11).

I can't find anything about this situation in the help docs.

The member may have moved and not put in new address for PBS.  But, it is odd for a new member.  Just wait for a reply from the member. If the member never replies, let the book time out and be declared lost in the mail. Or, after waiting a reasonable amount of time, contact PBS to cancel the transaction.  If the member replies that the address is correct, I think you can remail it without paying more postage.  Someone here can probably tell you for sure.

There's a good help doc called A Book I Sent was Returned to me.  It's a little long to paste here, but I've included the link.  Among other things it says:

If the book was returned to you through an error on the requestor's part, you can ask him or her for an extra credit (or money for the postage) to re-send the book,

If the book was returned to you through USPS error, you can take it to the PO in the packaging in which it was returned, and explain the situation - that it was returned in error from a current, deliverable address.  They can check the address and if it is correct and the book was returned through USPS error, they can send it again "under cover" from Post Office to Post Office, without requiring more postage.

It then goes on to give instructions on how to do each of those things.

The PO has been known to return mail undelivered in error. I have a friend here at PBS who I regularly send a specific dog coupon to and one week the PO just returned it .......was concerned she had had moved and not informed me but no, no move, and she verified the address I had on the envy was correct so it was just their mistake.

Give the requester several days to veryify the address.  If PO returned the package in error, you can take it back to them.  If the requester is in error, she can buddy a credit to cover re-sending postage.  If she does not reply (or other problems) contact BS admin.