The other thing is, if you did hand them over, as the author you can file a take down notice against the books if they have already been published. They were effectively stolen from you, and as such, if they have been published, that's piracy, and a take down notice should get the books pulled.
I'd love to be able to do that, one issue I had was finding the books, plus they've been published for years. I did find one (the post apoc series) and I tried contacting the author/publisher through the contact on their author bio, which led to a Facebook account. I got no reply, but the guy's name is different from the one who contracted me, so I got worried i could have been working for a middle man, and the author/publisher paid them already, they just didn't pay me.
I haven't found the second series yet.
The same guy actually brought up a dispute on the freelance site where I got contracted for the books, trying to take back payment for another book I did for him. I didn't know and only figured out later when my account was shut down. I did manage to contact their support for temporary access to the account, and I have screenshots and html files of all our chats, including the contracts for the books and all the original files, but I can't do anything with them. I guess I felt a bit of guilt because I didn't want to bother people who weren't involved in the dispute. I don't think it's even the first time this guy tricked his clients.
At first I figured I could fix the books up, do some edits and publish since it's been a while, but I don't want to risk my kdp account. Even with all the proof, there are plenty of horror stories of authors having bad experience with Amazon support, and I don't want to be blocked. So I'm selling a few copies of the plots instead.