I'm a backend engineer, Google GDE, working on Kotlin for 5 years. I am writing a book on how to use functional programming to build a complete application in Kotlin. I started working on it 3 years ago and it's based on real production code of several projects I worked on.
It's now possible to buy the ebook in Beta:
https://pragprog.com/titles/uboop/from-objects-to-functions/