The climax of a story is the moment of greatest emotional intensity, when we have been led to a high point through the rising action to a significant moment of suspense. In this novel, the climax comes at the end of the novel when Liesel's world is literally blown apart by a bomb raid, and all that she knows and everybody that she loves (almost) is killed:
In the space of a few minutes, all of them were gone.
A church was chopped down.
Earth was destroyed where Max Vandenburgh had stayed on his feet.
Death is shown to move around all of the people, friends and family of Liesel, claiming each one of them. However, Liesel is saved because she was in the basement of her house, and thus she is pulled out to face another day and does not meet Death ultimately at this point in the story. She is left to be looked after by the mayor's wife and to be reunited with Max at the end of the tale.
The climax of a story is the part when there is great and intense emotion, built up through the rising action and often with great suspense. In The Book Thief, the moment of intense emotion is when Liesel's street is bombed and most of the people she has grown to love are killed. The place that she has come to call home is ruined. During this time, Liesel eluded death once again because she had been in the basement during the bombing so survived the attack. For a time, Liesel feels that her world has ended and that she has been left behind, but at the end of the book Liesel meets back up with Max, and it appears that her life has a purpose and meaning again.