Zettelkasten method of note taking increased the writing output and quality of Nicklas Luhmann significantly. He maintained his zettelkasten for all of his professional life - approximately 40 years. In that time he processed through and wrote about 90,000 notes - an average of 6 notes every single day.

With those notes, he was able to pull from his notes and turn them into manuscripts. He wrote 58 books and countless articles in his lifetime, an average of two published books per year of his professional adult life - see: Following interests still needs discipline.

My claim is that Writing in a zettelkasten is pre-writing future books.