I tried to look back through the various Zettelkasten threads here, and found that most of the intro ones were a couple years old.
I’m interested in the topic but don’t really feel that I understand it well.
I’d like to learn the theory separately from “Here’s the theory and this is why you should use my/our app”.
Are there good resources out there for that? This topic is too popular to be easily googable any more.
TIA.
I’ve just finished Sönke Ahrens’ How to take smart notes and consider it a good theoretical and technical introduction to Zettelkasten.
The Archive is also a good resource, filled by long and detailed blog posts about Zettelkasten, although it’s a bit fussy from my perspective.
Just picked up the Sonke Ahrens book on kindle. It’ll be one of my Christmas break reads.
I think the information here
is very extensive and very structured.
It helped me get started last year, and I still go back to it every now and then.
If you want to go back to the beginning, read Niklas Luhmann’s Communicating with Slip Boxes, and decide for yourself how your want to emulate his methods.
I like this better than the Ahrens book, which I found recursive and sorely in need of editing.
I’ve learned that Zettle is one of things you just have to do for a while before it starts making sense.
– and follow your own path. For me that includes:
Software not required 
I’ve been learning and working the method for over a year now and I think Ahrens’ book is not a good introduction. It focuses way too much on academic workflows and the method is actually more flexible and less focused on referencing than it appears in many places.
The most synthetic and clearest write up I’ve read so far is this one.
I disagree with some details there in my own practice but it sums up what the method is and what it brings very nicely.
I read the book, and I enjoyed it, but it’s because I never have really been presented with a good system of taking notes in one cohesive document like that.
I think so many of the guides are so complicated that I feel like I’m always missing something, like it needs to be more complicated than it is, and maybe I’m simplifying my guide too much here, but here’s kind of how I view it:
You need two buckets (or at least think in two different buckets). Your notes and your reference manager/notes.
It seems to be way more about what you learned as opposed to what is exactly presented.
I think the organization part was very specific to the creator of the Zettelkasten method, that it’s easy to get lost by replicating it exactly using the tools we have today. Find some software that allows you to link ideas together and you’ll be most of the way there with a fraction of the work the original slip box method needed.
Last night I found a lecture on Vimeo:
Sönke Ahrens - How to take smart notes
And watched that.
He’s so focused on a system to publish that I didn’t find it overly helpful after getting the basics down.
I believe that @Aaron_Antcliff is on to something here:
That makes sense to me.
My revelation was that the two buckets are information and knowledge. Zettelkasten is an overarching, structured, and nuanced method to manage the latter.
—
JJW
OK, maybe I’m a bit slow today, but… how are information and knowledge differentiated?
My epiphany was similar, but I viewed it as resources and notes. Resources are the primary source and notes taken while viewing the source, then the notes are what it means to me.
Since you’re a preaching man, I kind of thinking of it like this: taking notes while reading The Bible and any commentaries would be the resource. You make notes on what you read, and you attach it to the source in a reference manager of sorts.
Then you take your notes ok your reading and synthesize the information, sometimes we call it a sermon.
This synthesized thought is the note.
That’s my take on it at least.
It is good to have all of your original notes attached to the primary source, but it’s even better to capture the idea somewhere else.
One of the readings (that I cannot directly find again) stated that information and knowledge are simple to distinguish. It then went into detail about information but said nothing about knowledge. 
Based on that reading, I would call information a collection of stuff. Based on further reading between the lines there and elsewhere, I would say that knowledge is the assembly of the context and meaning that makes the information relevant to you.
ZK is not just an index of what you know, it is a map of how you understand and relate to what there is to know.
I might hazard a guess (roughly following a Blume taxonomy of mastery) that the next step is to apply the knowledge. We don’t apply information, we apply knowledge. To gain knowledge, we must relate information in a way that takes it beyond just being a collection of stuff.
—
JJW
This recent article was about writing a book; the author used Zettelkasten and gives links to other material
Ah! Now I get it. Helpful analogy, as was @DrJJWMac’s comments.
The simplest example I can think of is this:
Imagine I have a friend named Andrew.
In my Contacts app, I have an email address, work number, and cell phone number for him.
I know from experience that Andrew rarely responds to email, spends lots of time in meetings so calls aren’t always possible for him to take, but he happily replies to text messages very quickly.
Andrew’s phone numbers and email address are information.
Knowing how to best communicate with Andrew is knowledge.
To me information is a motley collection of stuff that relates to something. Knowledge is how to incorporate that information to make informed choices, perform actions and use that as a base from which to expand the sum of human knowledge.
Example:
Information is a book on the history of scientifically based sheep performance recording starting back in the 1600s and several research papers on pedigree analysis and using measuremtns fo gene interations to predict performance in offspring with dates starting in 1922 and up to this year.
Knowledge is me taking that information, applying it along with accurate measurements, genetic DNA results and statistical analysis based on linear equationsand BLUP methods to select the best sheep for my environment and my flock goals. And from there to fine tune the selection process by adapting what data I use in the analysis.
I’ve been using it with Obsidian for several months now and it just beginning to make sense. All I really know is that if feels good to express my own thought in the note. I usually just think in my head about the thoughts stimulated by the source reading and end up losing the precise thoughts and trying to recreate the thought pattern with great difficulty when I get to writing about it.