Getting Things Done
I was sharing about my old getting thins done (GTD) setup just now. And it's actually quite impressive how much notes I've written in obsidian and how elaborate it.
Here is my obsidian graph:

I got really into this second brain method after first getting exposed to it by Ali Abdaal. I saw his recommended book on Building a Second Brain and some of his youtube video on how he manage his notes, and I was impressed.
Then I searched on what is the best book to introduce me to this and got into 2 recommendation.
Building A Second Brain (BASB) by Tiago Forte & Getting Things Done (GTD) by David Allen.
Personally I recommend BASB over GTD. Mainly because GTD is based on traditional method aka (physical files and paper), while BASB is more relevant to current times.
If you have time to read both, it'd be the best tho.
Then BASB and GTD itself only present the system, but how to implement the second brain and what tools to use is up to the people. I personally found obsidian to be the best for me as dev. I really recommend Nick Milo and Aiden Heart content, those are what I use to setup my own obsidian.
Personally I recommedn reading BU