I think that Liesel consistently steals books as a way to capture and get the words back in a sense. She also wants to learn how to read books and is something she loves to do with Hans. Also, early on in the book when Liesel is having the nightmares, the books are a way to comfort her. On page 64, during the night she has a nightmare and Hans comes in to comfort her and sees that she has the Grave Digger's Handbook, he says "Is this yours? Yes, Papa. Do you want to read it? Again, Yes Papa. A tired smile. Metallic eyes, melting. Well, we'd better read it then." She sees reading as an escape from her nightmares and enjoys the comfort from Hans every night.