I'm not such in favour of books when it comes to the latest API's and especially APIs that might be experimental in some parts or still not stable.
Once you buy a specific language book, it's automatically outdated even though you might learn tones of stuff.
What never gets old regarding books are books that talk about principles of software, maybe c++ books etc.
As for coroutines, I would advice you to stick to the documentation for 2 main reasons:
1 - You get discipline of work (read the docs - implement, repeat)
2 - Kotlin docs are just enough, IMO they are perfect for beginners, intermediate level and extremely experience programmers.
Also, watching conference talks and asking for help in the community is strongly advice. Obviously I think you already are doing that one