Post Office delivered empty, open media mail

I ordered a textbook online and received an empty envelope.

Apparently the glue on the envelopes is crap because the envelope was open and it obviously had the book inside at one point, because the corners of the book had ripped holes through the envelope.

The postal worker had the nerve to scan the barcode to confirm delivery...OF AN EMPTY ENVELOPE!

I know I can prove the the post office that it was empty when I received it, because if it had the book inside, he would have needed to leave it in the parcel boxes, and leave me a key to one of those. Surely they keep record of who they leave keys with, right?

I don't know what to do!!! I got a great deal on the book, almost half off of the price of a used copy at the campus bookstores. So now I'm out 56 bucks and need to go buy a new textbook for $101, that's assuming the bookstores have any used copies left, or else I'll have to pay the price for a new book which is outrageous.

Anyone have experience with this?!! I was never given the option to insure the package, I suppose that would be the sender's responsibility??