According to the help center:  Lost books must be marked received when/if they arrive, to keep your account in good standing.  That being said, I would wait until the book officially goes lost, then PM the person with that info (try to find a nicer way to say it).  It's a last ditch effort, but it worked for me when I was in the same situation.  If it still goes unresolved, I would probably not involve R&R since you did get your credit.

It seems like this person could at least answer your PM's and say yes they have it or no they don't.  If your DC says it was delivered, it was delivered somewhere.  Yes the DC's can be scanned along the way, but in my experience they don't say delivered until they reach the post office/house of the person they are being delivered to.  It seems like if they don't have the book and the DC says it was delivered, they would be wondering where it got delivered to.  Maybe they are out of town, but it has been an awfully long time for someone to be away. 

Shannon, if I were you, I'd just let it go. You've got your credit (thanks to PBS postage), and the book has been scanned in transit so there is proof that you mailed it. I just wouldn't worry about it. If the other member lets it go lost, well, so be it. You're in the clear.