Observables are known for having a steep learning curve due to the fact that it requires a different way of thinking. Here are some helpful topics about working with and understanding reactive programming using the observable model.
​RxJS 5 Observables Reference - Reference material on RxJS 5 Observables. There are many breaking changes from RxJS 4-> 5 so please use documentation on version 5.
​RxMarbles - Quick references for visualizing observable value fulfillment through the use of marble diagrams