Is there a “ground-up” explanation of PGP/GnuPG?

[Chris]

I have asked this on HN[1] as well as Reddit[2] too, but I realised you
people might be a better audience for the question! (...And it gives me
a good excuse to subscribe to my first mailing list!) Question below:

Understanding how git works internally "from the ground up" has been
incredibly helpful in my everyday work; things like blobs, commit
objects, hashes and how they connect to form the git experience as I
know it. Where I had been cargo-culting along previously, it all became
clear once I understood the fundamental model of what was going on
underneath the interface.

I feel like the same thing could apply to PGP/GnuPG. I am cargo culting
my way along but I feel like I would feel much, much, much more
comfortable if I knew how it worked from the ground up.

I have loose ideas of asymmetric cryptography and trust circles and
such, but nothing concrete to hinge my actions upon, so I mostly try
different permutations of command line arguments until GPG appears to do
what I want it to do.

Is there a "from the ground up" good guide to PGP that allows me to
break out of this pattern?

[1]: https://news.ycombinator.com/item?id=13070261
[2]: https://www.reddit.com/r/GnuPG/comments/5fpfgy/crosspost_from_hn_is_there_a_groundup_explanation/