The RxJava2Interop library can be included in the project by adding the following line:
compile "com.github.akarnokd:rxjava2-interop:0.8.3"
In the project's build.gradle file under dependencies, there might be a newer version of the library by the time this book is released so that the reader can check whether that's the case by visiting https://mvnrepository.com/artifact/com.github.akarnokd/rxjava2-interop.
The https://mvnrepository.com/ website is great, in general, to check whether there are newer versions of the published libraries that are used in the project.