World War Two (WWII), also known as the Second World War, was a global war that lasted from 1939 to 1945. It was the most widespread war in history, and involved more than 100 million people from over 30 different countries. The Second World War resulted in approximately 50 million to 85 million deaths, making it the deadliest conflict in human history.
WWII altered the political landscape and the effects are still being felt today.
English Heritage has a range of books discussing the Second World War for your perusal.