What is the author's message in The Book Thief ? What is he trying to get across to the reader?

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.