Ordered two books in one shipment, package arrived with only one book. Messaged the sender and asked if the books were coming in separate packages, he said no, they were mailed together. Pbs is telling me to mark the book as received because the package (and the assumption that it contained two books instead of one) was scanned at the post office. 

Do I consider the book not received as lost and mark it as received or let it time out? Haven't heard anything else from the sender other than he mailed both books in the same package. 

Were both books requested FIFO, or was the second one a freebie?  If both are showing on your incoming tab you should be able to mark just the one received and let the other remain not received...just let it time out/go lost if it doesn't arrive.

Did the packaging appear damaged?

Do not mark the book you didn't receive as received in any way, otherwise you will not get your credit back.

Even if they were ordered together, you will be able to mark them separately if you choose "problem with 1 or more" from the list of selections that you see on the Books En Route tab. Mark the one that you received as received, and mark the other one as "book missing from shipment". (It will stay on your Books En Route tab by itself until it goes lost, at which point you get your credit back)

Both were requested, 2 books for 2 credits. The package was in terrible shape. I have marked the one received as received, good condition. Thanks for the reply. 

This is not an infrequent mishap during shipping.......many threads over the years where members report getting an empty wrapper or only one book of a multiple order.   On a rare occasion the runaway book or books may meet up with the intended recipient, but more often than not you might be given something else that fell out of someone else's damaged wrapper!

Many people here recommend that multiples either be packaged in a box or make sure the envelope is wrapped tightly around the "brick" of books as when books slide around loosely, they are more likely to punch a hole in the wrapper or envelope and then "escape" ---never to be found.