I am unashamedly interested in optimising my own productivity – a key part of this is being able to absorb new content and retain it for future use. I realised long ago that reading in itself is insufficient; one needs to proactively seek to retain and contextualise new information. My solution was to make highlights while I was reading (the Kindle has been instrumental in this) and to put a little more effort into my written reviews – like this one – afterwards. This is a decent start, but it does not address the key task of making new connections and synthesising new content.
How to Take Smart Notes presents the zettelkasten note-taking system used by Niklas Luhmann, a prominent sociologist and prolific author. The system involves keeping two "boxes" of notes: one containing "literary notes", which summarise a resource (e.g textbook, journal article, book) and contain bibliographic details, while the other box contains your thoughts and ideas. A key point is that you should constantly think about how your ideas link with others that you've written about. This results in an ever-growing network of well-formed and nuanced ideas – much like a second brain. It is straightforward to then mix and match these ideas to form a piece of writing, be that an essay or a journal article.
What I like best is that the system is not dogmatic. Rather, it is based on intuitive principles that can be readily implemented either physically with boxes and cards, or in modern note-taking software like Notion. Ahrens emphasises that the end goal is to build a flexible system that works for you. All this said, the Luhmann system is very much geared towards academic/nonfiction writing and does not seem to be immediately applicable for people studying examination-based STEM subjects, though I will personally find it useful for my academic blog and perhaps any dissertation-related material in future.
EDIT (Dec 2021): this book was a key spark in the renovation of my note-taking system. It's honestly hard to understate how important this has been for my development.