If you mean WW II, you may have to go through many many books. Daunting task. But to get an overview of the whole world war, I suggest you can try World War II for Dummies by Keith D. Dickson.
Wkipedia may serve but there will be too much of going back and forth with the links and then keeping track of where you started and all. Will soon get to be too much - like doing a thesis. The dummies books gives you all on a platter.