This book describes the Zettelkasten method for better note-taking (i.e. the "slip box") where you take good notes so that you can easily pull from those ideas later for a better writing experience.
The inefficient way of note-taking is absentmindedly putting notes into folders where you will never see them again. The efficient way is giving the note context and linking it to other notes as you make it, so that you can better remember it.
The whole writing process (brainstorming, choosing a hypothesis, writing, and editing) is easier and more creative if you take notes on things that spark your interest IN THE MOMENT, because you then have a repository of networked ideas to browse through when you need it.
Only the first part about Zettelkastan. The rest of the book is obvious, common sense info.