I’m into Kotlin, because it’s a new (quite new) programming language, that solves all the Java problems, especially on Android. You can simply use Lambda expressions and much more on any API version. I also used Kotlin in production in my open sourced newsreader app NewsCatchr. Here I’ll share some of the sources, that helped me getting some knowledge in Kotlin:
The documentation The documentation is a really helpful source. I learned Kotlin myself through practice and every time I didn’t understand something I went looking in the docs and they already answered most of my questions, for example how to use any of the many helper functions in the standard library.