What commentary does Rudy offer about Liesel’s thieving? What does Rudy end up stealing? 

From the text:

“You know something, Liesel, I was thinking. You’re not a thief at all,” and he didn’t give her a chance to reply. “That woman lets you in. She even leaves you cookies, for Christ’s sake. I don’t call that stealing. Stealing is what the army does. Taking your father, and mine.”

Rudy seems to be saying that Liesel's habit of stealing books isn't really stealing, but rather someone inviting her to steal. Real theft occurs when something or someone you love is taken away.