Kotlin Coroutines: Deep Dive by Marcin Moskała is truly a great book on Kotlin's coroutines, which are pretty much the silver bullet for async/concurrent processing in Kotlin. Practical, comprehensive, and gives you insights into how things work under the hood so you can understand them better.
I went through it myself and cannot recommend it better.
https://kt.academy/book/coroutines
Whilst on the website, you can take a look at the
articles section for free, topic-specific resources, they have multiple detailed articles on popular coroutines (and Kotlin in general) topics.
And for a bigger picture of coroutines' what and why I would recommend going to youtube and watching Roman Elizarov's talks on them, they are exceptional IMO.