What purpose do books serve in the novel "The Book Thief"?

For Liesel, books are important as both material objects and as representations of the knowledge contained within them. Liesel, in stealing the books, is actually breaking the law ... an act of resistance. Ilsa's actions can be seen in much the same way. She knows that Liesel is taking books from her home, and she even encourages it ... she invites Liesel to take the books, and in doing so, she encourages her to learn about things that discouraged or forbidden by the Nazi regime.