Why do you think it was 'good' to steal the book vs. it 'not' good to steal the book? Why did Liesel steal this book, rather than give it back to the owner, in which was looking for the stolen diary?

I suppose you are referring to when Liesel steals the books from Ilsa Hermann. Liesel only had access to books by stealing them from Nazi book burnings. That was pretty risky. Liesel didn't think the Mayor's wife would lend books out, so she stole them. She also didn't think that Ilsa would even know they were gone.