Talking about Chapter 2, Observables in the RxSwift book. 

Discussing what are observables, how can we create them, and how to subscribe to them. Also talking about disposing and terminating. Lastly discussed Traits in RxSwift and the different kinds of Traits available.

This book by Ray Wenderlich acts as a complete guide toward learning and mastering reactive programming and asynchronous programming. This is currently using Swift 5.1 and iOS 13.

Links:

Step Into Swift: https://www.stepintoswift.com/

https://www.raywenderlich.com/books/rxswift-reactive-programming-with-swift