....on Sept. 27th.  This person had a regular street mailing address so I assume she gets her mail there.  Anyway, she claimed she does not yet have the book, when I PM'ed her to ask her to please mark the book received.  Is this possible? 

Very possible. It could have been delivered to someone else by mistake. DC just means that it was delivered - somewhere!

Leigh, I had this happen to me just recently with a swap book. It was scanned 'Delivered' last Wednesday, but I still haven't received it! And I'm a letter carrier!!

My regular carrier was off that day--no excuse, but a possible explanation for the mix-up.

The book that was missing just showed up in my mailbox today. I'll have to wait until later tonight to see if the scanner information has updated on usps.com.

One of the books en route to me shows that it was delivered on Oct. 2nd. It wasn't.