For the most part I found the official Kotlin reference documentation to be excellent and more than enough to move from a strong Java and Groovy background. I

For the most part I found the official Kotlin reference documentation to be excellent and more than enough to move from a strong Java and Groovy background. I follow /r/kotlin as well. Advanced idioms and stuff I've just picked up looking through GitHub repositories. What I really like about Kotlin is just about everything "works" as it should or you can usually write a Google search that will reference Stack Overflow. I don't have to dig much to find answers and in the absolute worst case I can just drop back to Java and do it the classic way.