Also like other Cathedrals, if that is all you notice, you've completely missed the point. A Cathedral is not just a building, it is an outpost of life in a world marching lock-step towards death. It is a refuge of light and love from the madness of a sin-sick world. Which is what books are for the characters in The Book Thief. Books become a means of rebellion, a way to fight against a world spiraling out of control. Even that very worst of books becomes a means of salvation and escape (though I won't tell how, that would ruin the story).
Ultimately however, also like a Cathedral, this book is a battleground. The place where two words meet and war with each other: the word of death and the Word of Life. One the one side is the word of death. So Max describes Hitler:
The Fuhrer declared that he would rule the world with words. "I will never fire a gun," he devised. "I will not have to." Still, he was not rash. Let's allow him at least that much. He was not a stupid man at all. His first plan of attack was to plant the words in as many areas of his homeland as possible. He planted them day and night, and cultivated them. He watched them grow, until eventually, great forests of words had risen throughout Germany.... It was a nation of farmed thoughts. While the words were growing, our young Fuhrer also planted seeds to create symbols, and these, too, were well on their way to full bloom. Now the time had come. The Fuhrer was ready. He invited his people toward his own glorious heart, beckoning them with his finest, ugliest words, handpicked from his forests. And the people came.The fruit of the seeds planted by the words of the Fuhrer were realized in the end of the book (which again I won't spoil), but were also seen throughout. The words of Mein Kampf grew into the war, and the ovens.
On the other side is the word of life. When this word is planted, it looks an awful lot like adopting an unwanted and socially unacceptable child (a child of a Communist, no less). This word grows with patience and kindness and becomes life and salvation. It feeds the hungry; it supports the failing; it hides the persecuted. Even when it appears that the word of death has won, it flourishes in hiding and wins through to the final reward.
Just as the Cathedral forces us to choose which word we will live under, so this book presents us a choice: life or death? The word of hatred or the word of kindness? The word of destruction or the word of salvation?
Read at the instigation of my wife; to whom I say: well done.