Here are a list of references provided by Dassault:
User Help–EKL References (Docs)
EKL Intro (PDF)
- EKL PDF By Dassault (2018X)
- This is a link to a PDF made by Dassault to cover the basics of EKL. It is made for 2018X, but it is still very relevant for earlier and newer versions of 3DExperience CATIA because it does not focus on release specific functionality, but the overall structure of EKL.