Spaced repetition is a learning technique where you embed things into memory by re-studying them on a regular basis – for example, organising flash cards so that each new concept is refreshed after a day, then a week, then a month, then a year. One of the unappreciated functions of many newsletters – to be clear, not this newsletter, but other newsletters – is to function as an ad-hoc spaced repetition system.
My favourite exemplar of this is Matt Levine, who writes an unreasonably fun daily newsletter about finance. Levine has about six key concepts that he returns to regularly through the newsletter, and as he discusses each day's news, he'll often re-present us with one of those key concepts (e.g. "everything is securities fraud") as it relates to a given case.
Because I get reminded of those concepts on a regular basis over a long period of time, they are far better embedded in my memory than if I'd read Matt Levine's Book of Finance over the course of a few days or weeks.
A funny part of this is that Matt Levine's work probably exists in newsletter form because he writes about "news", he's always responding to current events. But at least on the spaced-repetition side of things, this isn't actually important – he could be giving us a random historical example every day and the daily refresher on the core concepts would still make the newsletter a good delivery mechanism.
One thing I wonder about is which kinds of topics are better served by newsletters, and which are better served by books. Presumably if you're creating a complex argument that requires the reader to hold in mind various ideas that build together, a book is a better fit than a newsletter.
However, many books I read strike me as having One Big Point and then a long series of examples, and in that case I suspect a newsletter dribbling out the examples might be better for reader retention.
One weird wrinkle of our present age is that a single month of a paid newsletter susbcription can be more expensive than buying a book. I wonder if we're missing something like a "limited series book-by-newsletter" concept, or whether book purchases could come with a spaced-repetition series that reminds you of the key points over time.
We spend so much time reading that it always makes me sad to think that including a little more repetition would have disproportionate impact on our ability to remember and relate to the information we've read. In theory we could be note-taking and flash-carding after reading, but this (frankly) feels more like a chore than a pleasure. At their best, "repetitive" newsletters are one way to achieve the same goal less aversively.