Fundamental Kotlin is a book focused on Kotlin programming language and it’s comparison to Java. This book is focused on language’s most important features and aspects. Book doesn't go in too much theory since it's focused on exact examples with notes and code provided.
This book is for developers and for people who are on its way to become developers. It is essential to have at least some basic knowledge of computer programming, especially of Java. Fundamental Kotlin is imagined as guide to Kotlin for developers who spent some time on Java powered projects but it is not mandatory. If you were programming for some time, it should be easy for you to follow this book.
A reference example
This book is exactly what was missing in the Kotlin literature so far. Brief, concise and quick reference flooded with almost always great samples. Not sure how to use closures? Here's the answer. Not sure about thread safety? There's the answer too. What about generics, or type casting, or control flow enhancements? Bullseye, you'll find it within seconds. This IS NOT a book from which you will learn how to do the basic programming stuff. This IS a book to help you switching to Kotlin seamlessly, if you have any relevant expirience with Java, Objective-C, Swift, or Object Pascal (Delphi). Highly recommended.