Hello Aspirant,

I'll recommend you to refer to the study material provided by institutes of company secretariat of India. Candidates are always advised by the officials to refer ICSI CS Executive study material on priority for both old as well as new syllabus. Once the CS Executive study material  is exhausted, one may freely revise with other sources and explore other dimensions of the same topics.

You are advised to visit the following link for more help:

https://finance.careers360.com/articles/cs-executive-study-material

all the best