"Number the Stars" by Lois Lowry is a great story about a young girl.
"Night" by Ellie Wiesel
"Slaughterhouse Five" by Kurt Vonnegut
Those three are listed in level of difficulty.
Child of Hitler, by Alfons Heck. He was an actual member of the Hitler Youth, and talks in detail about how he was sucked into the Nazi way of life, worshipped Hitler, and then became disillusioned later. Very good book, from a primary source. You can probably only find it on the internet.
I did some more research, and other good WWII books are;
"The Bridge over the River Kwai" and No-No Boy.
And there is always the classic, Anne Frank: The Diary of a Young Girl.