The description of the observer design pattern was originally documented in the book Design Patterns: Elements of Reusable Object-Oriented Software. You can read it to understand one of the foundations of ReactiveX. This book contains the description of all base design patterns used in object-oriented programming.

The ReactiveX documentation is available online here: http://reactivex.io/documentation/operators.html. This documentation is generic to all programming languages, featuring marble diagrams of each operator. Refer to this documentation when looking for an operator. For each operator, it contains links to the specificity of each implementation.

The Reactive Manifesto is a great source of information to understand what is a reactive system and how to implement such a system: https://www.reactivemanifesto.org/.

You must read and learn The Observable Contract available here: http://reactivex.io/documentation/contract.html. It describes in a few words the rules that govern observables and observers. Once you are at ease with these concepts, you will be able to write and read ReactiveX code in an efficient way.