Another great find. Thanks!

Reading the card system chapter makes me think it must have been easier to learn these methods directly in person than from a book. For example, the sugggestion of using symbols on the top corner of the cards is intriguing, but I don't find it obvious how you'd use them.

Also, this is further confirmation that Luhmann's approach to linking notes by means of a unique reference ID (a kind of proto-hyperlink) wasn't common. This book doesn't mention the possibility.