Unfortunately, I foresee this being a long, arduous process that may not resolve completely in your favor. Your options are either find a completed sale on eBay or another site for the book in the same grade or perhaps an appraisal from a dealer would suffice. Either way, I think USPS will try to give you a hard time regarding the proof of value under these circumstances because they suck.
Sorry this happened. It won't help in this situation, but a personal Collectibles Insurance policy would be much easier to collect from than USPS.