The problem with researching WWII is that there are a lot of books written on the war. So most students will use these books. While there is nothing wrong with that for an overview of the war, but if you want to encourage students to think critically of the war, if student are using only secondary sources someone else is influencing their thoughts toward the war. So I found this website that is an index of primary sources of WWII.
When I taught WWII as a teacher I tried to use only primary sources so students could make a clear choice about the war in their opinion.