on its way to the sender (say the book gets rained on and with no plastic covering over the book, it gets wet) whose fault is it? The sender or usps?  Or no ones?  

If I feel that the packaging was adequate, then I'd mark it as damaged by usps. Adequate to me is nothing more than completely covered in paper and secured along the seams with packing tape.

If I think that the wrap-job is poor , and it was damaged because it came loose or opened, then I'd mark it as damaged by sender.

ETA: Plastic wrap is not a requirement. Mail does sometimes get rained on. If you live in an area with a lot of rain, you might want to consider asking for it by using the RC feature in your settings.

If the book is damaged in the mail, whether or not the packaging is adequate, it is properly marked "Damaged by USPS".

Damage by USPS is generally considered the fault of the USPS, and what happens is that the sender keeps the credit, and the receiver keeps the damaged book. (This is the fairest way to resolve the issue, because the damage is certainly not the senders fault, so they should not be penalized, and the receiver usually gets a book that they can at least read, if not repost (so they do not suffer a total loss)).

So, normally when a book is damaged in the mail, the receiver does NOT request their credit back, and the sender is certainly under no obligation to refund it.

However, PBS does provide that if you feel the packaging was inadequate (and this means that they used scotch tape instead of packing tape, or some other clearly inadequate way of packaging) .... then the receiver can request their credit back in that case. But you don't do that based on a personal feeling that the regular paper wrapper is inadequate for books, for example, because PBS clearly recommends that the paper wrappers be used. And, PBS also does not require a plastic lining, so that is also another case where not including one does not mean that the book was inadequately packaged.

There are really only 2 things that the USPS can do to ruin a transaction (either damage the book, in which case the sender keeps the credit, OR lose the book completely, in which case the receiver gets their credit back). So, over time, this all works out to be fair to both senders and receivers, for damage or loss that is not within anyone's control.

If a book is water damaged on the way, it is the USPS's fault. The USPS should protect the mail while it is in their possession.

In the PBS system, the risk for USPS damage is split between the sender and the receiver. If the book arrives at its destination damaged by the USPS, the receiver suffers the loss of the credit with an unpostable book (though they can offer it in the Bazaar). If the book is damaged and doesn't make it to the receiver (like an empty envelope showing up), the sender suffers the loss of the credit and book. In the case of water damage, the book usually shows up damaged at the receiver's so the receiver is usually the one who suffers the loss.

Thanks for the answers.  It was more a hypothetical question than an actual issue, but now I know.

As a Thought For the Day Tina, it's an interesting one. 

Water damage enroute, yes that should be damaged by USPS.  It would be difficult to think of a circumstance that the sender should be blamed for that.  At this time, USPS guidelines do not require plastic underwrap...so not using plastic cannot in itself incur RWAP.  Couple years ago I received a book in a wrapper that at one time had been very wet.  There were no enroute scans, but somehow the book had been delivered anyway (despite the fact all the printing had run, was blurry and/or unreadable, and the DC was unusable).  I finally decided that the sender had dropped the book in a leaky blue box (before torrential rainfall).  Luckly the sender had used plastic underwrap and the book itself arrived in pristine condition.

Requesting members that have potential water damage situations (heavy rainy season, walking mailman with open mail pouch, leaky mailbox, etc.) will often have an RC requesting plastic underwrap.  In those cases, if no plastic is used and the book arrives with water damage, the receiver can RWAP the sender directly.

But I would say there is a possibility of enroute damage that could (in theory) be charged to the sender.  Just for the sake of discussion (and did we actually see this one posted?) let's say a sender cut out the shipping portion of the label, scotch taped it to the book itself and blue boxed it.  If the back cover gets torn off enroute...how to mark it?