Really good stuff. Niklas Luhmann is my hero. In this book, Sönke takes you through the concept of Zettelkasten - a way to approach notetaking and personal knowledge management.
Niklas Luhmann was a prolific writer and was able to do so because of his notetaking method he called Zettelkasten. By capturing and linking notes and ideas, he was able to have many pieces of writing on the go at once and able to follow whatever interested him in the moment. I aspire to implement this system in my own life with Roam and this blog.
Also check out Ryan Holiday and Robert Greene who have similar methods.