The Cloud Security Alliance wrote the "Security Guidance for Critical Areas of Focus in Cloud Computing v4.0".
This document, the "CSA Guidance" for short, is the single most important document to read if you want to pass the CCSK exam. However, it is not necessary to read it now if you follow the course, so right now you should just download it. In later units you will be referred back to domains in this document.
You will need this document (and the ENISA one) during the exam, and you should by then be prepared to search in them through a proper PDF reader that allows searching for phrases, not just words.
Version 4 of the CSA guidance was released in the fall of 2017.
Version 3 is available here just as an optional historical reference.