Have you tried reaching out to the buyer to explain that the package is on its way and should arrive to them soon? Sounds like he was a bit quick on the trigger with the claim, but the way eBay's "ask the seller a question" walkthrough is set up these days, it's very easy for a buyer to get shuffled into filing a claim. I've been getting at least 1 claim a week filed for the last 2-3 months, not because I'm negligent, but usually because somebody wants to know the tracking number on their package. It's unfortunate, but it's not the end of the world, and simply having a claim OPENED against you doesn't count against your record (not resolving it and having eBay rule against you will though).
My guess is that, once the package arrives, the buyer will either drop the claim, or forget about it until eBay sends them a reminder to close it out. Chances are, if the guy is buying a book from you, he's not a scammer (just speaking from experience, most scammers I have run across do not appear to be very well-read
). I hope it all works out!