1. Getter Mario's forum. Members like Judex there I think can help but he speaks French. I can convey the message to him but he hasn't been on since months. I can try posting your request in the Devilman section of the forum, but it seems that people generally are more into Grendizer there (since it's French) than Devilman, and I don't know if there are any Devilman experts there.
2. The wiki. I assume you've already spoken to Jfgslo about this. Could he help you?
3. The tumblr Devilman fans. I don't have any contact to them, but they're the ones to come up with the "My father died" meme. Maybe one of them got the book? Who knows.
4. HappyScans and Dynamic Pro Scanlations. They scanlated Devilman Lady, Amon, Devilman etc... Maybe they know something about it? I think you have a good chance asking them since they scan their own raws etc.
5. 4chan /m/'s board. The only place I know where people regularly talk about Go Nagai. They're more on the mecha side, but sometimes they talk about Devilman too.