I bought it on a whim after somebody mentioned it and was very pleasantly surprised. Ahrens makes a convincing case for use of the zettlekasten, but the book is mostly about how to acquire understanding of something. You could read it without having any interest in making a zettlekasten and still get a lot out of it.
Having read about zettlekasten online I thought they were one thing, but after having read this I feel that I have a deeper understanding of them, and how to approach using them. The goal isn't necessarily to archive all your knowledge, it's to facilitate insights and new ideas. In a way, having your knowledge and ideas cross referenced is just a nice side effect. Prior to reading this, I would have taken an approach that would have just been a database of things I learned, which takes a lot of time to create and doesn't do much for you in the end.
It's a short, easy read at around 150 (small) pages and the author keeps it pretty interesting. I plan on rereading it pretty soon.
edit: I forgot to add that a lot of the book goes over study methodsmost people are taught growing up, and why they don't really work that well. I thought this was pretty interesting as well.
If you haven't already, you may take a look at networked thinking applications, like the open source logseq[1] or all the similar proprietary outliner alternatives out there.
There is a growing ecosystem based on a modern note-taking style heavily based on bi-directional links for organizing your knowledge with a bottom-up strategy, and a friendly community sharing advice while discovering the possibilities of these tools.
Plus, you can do zettlekasten or GTD on these tools if you have already built a habit on these techniques. The outliner will accomodate that habit, and let you grow it long-term into a personal knowledge database providing opportunistic insights.
