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When I'm buying a book on a programming language, this is usually to have a good, centralized source of information on that language and not to have to look everywhere to learn about it.
The reliability of information in this book is unquestionable since both authors are developing Kotlin, and fortunately they have a good didactic approach, which is not always granted with language "creators".
The book will take you through the bases of the language 1.0, and is rather comprehensive by first justifying its existence and explaining the philosophy, then proceeding to the introduction, brick by brick. There is a small part at the end related to building the bytecode with tools like Maven and Gradle, and the general ecosystem, but it is very succint.
Even though the book is well-structured and didactic, some complex chapters are not very well explained, the collection ranges and constructor delegates, for instance, are very obscure and let the reader down. Sometimes too, a key ingredient to understand a feature is only introduced later in the book.
So as a result, it failed to be the central source of information I was expecting, at least for advanced features.
I didn't really find a better alternative to suggest. The language website is more complete regarding features like coroutines, the multiple targets, and the ecosystem, but it is terrible as a learning medium. To take the same example as before, the collection ranges (and iterator/iterable concepts, not an easiest notion in any modern language) only get a confusing and partial coverage, and the reader is left alone to learn those features by trial and error.
This language is still evolving, but unlike others, it lacks both a good up-to-date tutorial and a reference books. "Kotlin in Action" is almost the former, and most certainly a good introduction for the main concepts. After that, expect to be hunting down the information for more advanced features.
Websites like https://exercism.io/tracks/kotlin, https://kotlinlang.org/docs/tutorials/, or applications like the Edutools plugin of IntelliJ will be necessary complements in that endeavour, as well as the few blogs and videos on the subject.