When you ask for something, you say 'please'. It's implied here that the person is asking for 'medium to difficult' books although it's not actually phrased as a question.
Although, I don't think it would be a problem to say 'thanks' because it's also good to say thank you after you've asked someone to get or do something for you. Both are probably equally fine. I don't think 'bitte' is ever translated as 'thank you' although a native speaker would have to confirm.