When we talk about "take home messages" as they pertain to literature, what we are talking about is themes of the novel. This novel has several.
One is the value of literature. Leisel learns quickly the power of knowing how to read and write - and learns late at night with her foster father, Hans. It is as she gains these skills that Leisel develops more courage and strength as an individual.
Another theme is the power of humanity to come together in a time of crisis. The book is set before and during WW2. Leisel's own parents give her younger brother and her away to a foster family in order to save their lives. The foster parents take Leisel (her brother dies) in and even show her love and worth - despite the fact that she is not related to them. Later, they house a Jew in hiding because he is a man who saved her foster father's life in WW1. Despite the constant threat of death, this family is one that courageously chooses humanity and life through the risk of their own death.
Another prevalent theme is the overall idea of the inevitability of death. The book is told by Death as the narrator. Many of the characters die throughout the book - but through this point of view - loss of life is not necessarily mourned nor is death glorified. It is simply presented as a reality of life. Because of this perspective, though this is largely a "war book" it is not really seen as a tragedy.
Zusak posits several messages in The Book Thief, most prominently the message that humans have the capacity to be resilient and compassionate. Throughout the novel, the characters are faced with incredible obstacles that they manage to overcome because they have developed compassionate relationships with others. Hans Hubermann is a pinnacle of compassion and morality in the novel as he risks his and his family's lives to maintain his moral beliefs. He believes that Max has the right to his life, and therefore, agrees to hide him in the basement. Likewise, he offers food to one of the marching prisoners to appease the man's suffering even though Hans knows that he will be punished. Liesel respects and loves Hans for these actions. She also remains resilient through the novel, and skirts and alters her own sense of morality to suit the times. She is such a strong person that even Death must stop to take note of the little girl's story.