The Book Thief is a work of historical fiction. Its characters and plot are fictional, but they are interwoven with World War II. Real events such as the Battle of Stalingrad are watershed moments in these fictional characters' lives. Even Liesel and Max's transformative friendship is driven by the Holocaust, which forces Max to hide in the Hubermanns' basement. As is the case with most historical fiction, Zusak's book makes the point that history isn't abstract and impersonal—it's made up of personal stories.
In particular, Zusak's historical fiction is, in many ways, postmodern. Postmodernism refers to an artistic and literary movement that emerged in the post-war era of the 20th century. Modernist literature had previously experimented with perspective, flashbacks, and other formal elements to get as close as possible to representing the truth about the world. Postmodernism responded to the traumatic wars of the 20th century by challenging the idea that humans could ever make sense of the world. Postmodern literature uses similar formal experimentation as Modernism, but it uses such techniques to embrace ambivalence and confusion. Zusak experiments with form in order to break down broad, overarching narratives people have typically used to make sense of World War II. He is clear that Nazi ideology and politics were abhorrent and dangerous. He also challenges the idea that it is possible to tell a neat, linear story about the war and all of its trauma. Many characters in the novel are both victims and perpetrators of harm, leaving them difficult to categorize. Flashbacks, flash forwards, soliloquies from Death, and other stylistic choices help Zusak push against the notion that anyone knows where the story of World War II starts or ends, let alone where it goes along the way.
The novel is also a bildungsroman, or coming-of-age story. A bildungsroman centers on a protagonist who must overcome social challenges, form an identity, and establish a place for themself in the world. The protagonist is often young when the book starts, and the plot follows their life into young adulthood. Charles Dickens's David Copperfield and Charlotte Brontë's Jane Eyre are two examples of this type of story. The Book Thief covers Liesel's preteen and early teenage years. While its main action does not follow her as far into adulthood as some other bildungsromans follow their protagonists, this seems to be because Liesel must grow up fast: by the time she is 14, nearly everyone she loves has been killed. Within the short, five-year time frame of the book, she endures no shortage of character-building challenges. She learns how to keep a secret when it really matters, even from her best friend. She learns that no matter what propaganda is forced on her, she will never hate Jewish people. Most importantly, she learns how to read and write. Like both Jane Eyre and David Copperfield, she eventually uses this skill to record the story of her own life. Totalitarian regimes aim to suppress free speech, so Liesel's choice to become a writer is a defiant act. It signals that she has grown up and developed a mind of her own.