As others have said, it is a shame this has happened and I have interacted in some way with both parties, either in the marketplace or through another forum here. All good interaction. I am no lawyer and don't even pretend to be one here or after staying at a Holiday Inn Express but I do not see how either could definitively their argument as to where the damage occurred. I personally could not vote one or the other.
I do feel that both of you made hasty errors by submitting a positive response either from David when the book was first received or Eric when first asked about the damage and return. In hindsight, it would be best to ensure that you review all and ponder a response before writing something. Kind of like the "say it forget it, write it regret it" saying.
Good luck and either way, it is too bad such a nice book was damaged in any regard.