"I did 14 years with good behavior because of teases like you."
Bastian is curious about a book that the old man is reading, and he tells him the book is not for him because it's "special." So when the guy gets a phone call and leaves the book unattended, the little bastard nabs the book and jets out the front door. But it's OK because he leaves a note.
So, right off, the questionable morals of this movie start to come into focus. Our hero is never punished for this act (there would be no adventure without it). Remember, that old man didn't run a library. He owns a book store. And judging by the fact that there are no customers, we're guessing that he's not making that great of a living to begin with, so every sale counts. And that's aside from the fact that he was halfway through reading it when this little pecker pulled a smash-n-grab.