USPS is supposed to keep packages safe and dry, and they do a really good job - but nothing is perfect!
Since no one can tell if a package will pass through a rainy/snowy area and get wet, we do recommend (but we don't require) an inner plastic layer when Wrapping a shipment.
If you live in a very rainy/snowy area or your mailbox is not very weatherproof, you can use Requestor Conditions to stipulate an inner layer of plastic for books mailed to you. Read more in How to Use Requestor Conditions.
However, if the wrapping was truly inadequate for normal USPS wear and tear (such as the wrapping did not cover the entire book), and thus may have contributed to the USPS damage, then the requestor can ask the sender to refund credit, since the damage in that case would have been avoidable, and not simple bad luck.
To read about marking an incoming book received with a problem, please see There is a problem with a book I received!.