Read "Aggression against Ukraine: Territory, Responsibility, & International Law" by Dr. Thomas D. Grant. And in case you don't read books, I interviewed Dr. Grant just a few days ago. Prof. Grant is an American lawyer who teaches at the Lauterpacht Centre for International Law, University of Cambridge. You can watch the interview here:

Aggression against Ukraine: Territory, Responsibility, & International Law 
https://www.youtube.com/watch?v=_AGZo69hN4o