Yes, If you want to study online and home i will suggest you can purchases Major kalshi classes book (https://mkcpublication.in/) . Here available all book for Defence Preparation.
Yes there are lots of online content available for CDS examination. If you have sufficient time then you should go with NCERT. If you don't have enough time in that case you can go with few dedicated books for CDS. Don't collect too much study material. Stay with minimum source. Practice as much as you can. You can purchase book from arihant publication or Diksha or upkar.
Yes, indeed there are books available online for CDS serving as a guide in terms of coverage of topics available on all popular Ecommerce sites. However, I would suggest that you also consult NCERT books for better grasp over topics like Indian History, Geography and Constitution. This knowledge would come handy in various competitive exams.