In May of 2018, I published this Back to Basics post -
http://rgalen.com/agile-training-news/2018/3/5/back-to-agile-basics
The intent was to refocus attention back to some of the original thinking and mindset of the early days of agile. Not to sound too nostalgic, but life was much simpler then.
I want to add to the list I shared then:
If you want to get back to the roots of agility I encourage you to research the following…
Extreme Programming - https://en.wikipedia.org/wiki/Extreme_programming
Heart of Agile - http://alistair.cockburn.us/Rediscovering+the+Heart+of+Agile
Modern Agile - http://modernagile.org/
GROWS - http://growsmethod.com/
And you might want to investigate how Spotify is implementing agile practices. Not putting them up on a pedestal but considering them a role model for learning the basics of agility in practice. https://www.infoq.com/news/2016/10/no-spotify-model