"OpenPGP for Application Developers" is now public. I did some proof reading on that, so I'm biased, but I think it's a neat and clear intro to OpenPGP . It's more on the conceptual level than nitty-gritty bit level formats or details of the cryptographic algorithms, but it should help developers using OpenPGP understand what's happening. The site is not quite complete, but a very good start and you can help make it better.
https://openpgp.dev/
https://fosstodon.org/@hko/111573842808340744