In an interview with Random House that can be found on YouTube, Markus Zusak says that both his mother and father lived in Germany during World War II. He grew up hearing stories from his mother about seeing a boy give an elderly Jew some bread and having been beaten for his efforts by a Nazi soldier. His father told him stories about people not wanting to fly the Nazi flag from their windows and doors, and how he also did not want to go to the Hitler youth camps. So, many anecdotes in The Book Thief are taken from historical events and stories told to Zusack by his parents and grandparents, who actually lived in Germany during that time. One can see this when Hans Hubermann gives bread to one of the Jews who are being marched through town. Hans is eventually caught and beaten for it in front of Liesel.
In Part Seven, in the section entitled "The Long Walk to Dachau," Liesel and Rudy are playing soccer when they think they hear cattle walking through the streets. This is exactly how Zusak's mother, who was six at the time, described the sound of Jews walking through town at the ends of Nazi guns.
"Papa reached into his paint cart and pulled something out. He made his way through the people, onto the road.
The Jew stood before him, expecting another handful of derision, but he watched with everyone else as Hans Hubermann held his hand out and presented a piece of bread, like magic.
When it changed hands, the Jew slid down. He fell to his knees and held Papa's shins. He buried his face between them and thanked him" (394).
The above passage is exactly as Zusak says his mother describes what she saw in Germany at the age of six. The parts where Rudy Steiner enrolls in the Hitler youth activities might be from stories that his father told him. So, The Book Thief incorporates first-hand experiences that give insight into German history.