Whether you are a professional programmer or a hobbyist, programming an Android app is an attractive proposition. Android firmly dominates the global smartphone market, meaning many potential users. All the tools you need are free to download and use and it's easy to get started as Android Studio will build you a Hello World app in a few minutes. What you do next is more difficult as Android is a tough system to master and to create a good app you need a feel for how the system works and its limitations. The good news is that Kotlin is a language that makes working with Android much easier.
Android Programming In Kotlin: Starting With An App shows you how common tasks are done in Android Studio 3 using Kotlin, pointing out where necessary how this differs from the Java approach. Given that you can't avoid Java altogether it also explains how Kotlin can work with the Java-based Android SDK. Although Android development seems confusing at first, it has patterns which mean that having done something once you can generalize the approach to components you haven't previously encountered.
This book doesn't set out to be a complete treatment of Kotlin or Android. Instead it focuses on the things you need to know to write an app with a single Activity and a UI, a good foundation for further Android programming. Particular attention is paid to creating the user interface (UI) because this is what you spend most of your time working on even if the app in question is sophisticated. There are plenty of examples in this book but they are deliberately simple, stripped down to let you see what is essential.