It is fiction. The story is a work of the author's imagination, set in Nazi Germany, and it tells a very moving and poignant story about a girl and her experiences during that dark time, but it is not based on a true story in a literal sense.
Well, 'The Book Thief' is definitely fiction. It creates a fictional world with characters and events that are imagined by the author to tell a compelling story.
It's fiction. One key way to tell is the use of a very unusual narrator - Death. In real life, Death doesn't narrate stories. Also, the specific story of Liesel Meminger and her relationships, her stealing books in such a unique way, are all crafted by the author. There's no evidence that a real - life person had these exact experiences during that time in history.
Well, to determine if the book is fiction or nonfiction, you need to look at elements like the plot and the nature of the story. If it's imaginative and not based on real life, it's likely fiction. If it's based on actual events, people, or facts, then it's nonfiction.