I was wondering if anyone has an idea of how accurate this is?  I have two books that I've sent that are about to go lost, both of which show as having been delivered  weeks ago.  It's not a big deal as I used PBS DC, but I've been very curious as to what happened to these books.

Delivery confirmation is just that - confirming that it's been delivered - somewhere. It doesn't necessarily mean it was delivered to the correct address.  I would PM the recievers of the books in question and ask them if by chance they got the books and forgot to mark them as such as your DC is showing them as delivered.

Delivery confirmation does not necessary mean the book has been delivered.  My local post office has scanned books as being delivered when they received the book and not when the book was delivered. 

I actually work for the USPS and know a little about how this works.  The parcel is scanned "accepted" when it reaches the delivering post office.  The mail carrier scans it "delivered" when it is delivered to the home or post office box (or parcel locker).  It is scanned "attempted" if the receiver is on vacation (has their mail on hold), or won't fit in their PO box or parcel lockers are full of other parcels and customer has to pick up from inside the post office.   

Why doesn't the USPS scan a package as having been delivered twice?  Twice I have had packages scanned "delivered", which I had not received.  When the packages were found and they were delivered they were not scanned as "delivered" a second time.

I think our local postal service scans them at point of delivery, ie in my driveway.  I've seen them do it.  If they are scanned incorrectly at the wrong address, that is a problem with your local post office.  Complain alot until customer service improves, and try for somebody above your local postmaster...such as County, State, or Regional.

I've made several calls over the years, our rural mail is frequently delivered after 6 pm.  Recently I've been receiving USPS surveys, so somebody is tracking local customer satisfaction.

It is not always accurate.  I have a book enroute to me about to go lost.  It shows as being delivered by the DC on Jan 8th. I haven't seen the book. If they delivered it then they delivered it to the wrong house.