I have found these two to be useful:
> https://github.com/lsmod/proof-of-history-explained
> https://www.shinobi-systems.com/primer.html
> https://www.youtube.com/watch?v=rKGhbC6Uync
(I know I know it's an hour long, but honestly watching 3 geeks having a good natured session contrasting their systems in real time is a joy)