What objects are given to the mysterious young man in Stuttgart to help him escape the city?

In the middle of the night, the young man is given a copy of Adolf Hitler's book, "Mein Kampf". Inside the book is an identity card and a key. He boards a train and travels to an address where he is told help is waiting. The house turns out to be that of the Hubermanns. He uses the key to enter the house without knocking and alerting the neighbors.