In The Book Thief, how does the key come to be in Max Vandenburg's copy of Mein Kampf?
The answer can be found in Part Three of this excellent book. In it we see that Max is met by somebody who has forged papers for him and designed an escape route to hopefully get him to a place of sanctuary. This stranger, who has obviously risked a lot to help Max, explains what he has managed to obtain for him:
"The card is in this." It was a book. "You should put the map in here too, and the directions. And there's a key--taped to the inside cover." He clicked open the case as quietly as he could and planted the book like a bomb. "I'll be back in a few days."
Thus we can see that the key happens to be taped into the front cover of Hitler's classic, Mein Kampf, because this is where the man that helped Max Vandenburg placed it. Of course, the way in which the book that expounds Hitler's plans and philosophy is used to help a Jew escape and reach sanctuary is grimly ironic. Presumably the man who helped Max thought that using this book would prevent it from being searched.