Sticking with the theme of warfare; bombing is an imprecise science. While the targets are usually intended to be military, you can never guarantee where they'll land. In this case, they've landed on a bookstore in London during 1940.
The bookstore has been blown apart and open, and the books have fallen from the shelves. That's provided an opportunity for this young man to do a little free-of-charge reading in the open air. He's making the best of a bad situation.