CityEngine Help is the main resource to learn everything there is to know about CityEngine. Each section begins with an overview page and it is recommended to read these pages first to get an idea of the content in that section.
Additional resources
If you are new to CityEngine, About CityEngine and the Essential skills tutorial are a great place to start. Also, the following links provide further insight into CityEngine:
The Python reference can be found in the offline help at Help > Offline Manual & Reference > Python.