"I will return the book to the library tomorrow."
Can anyone please answer my question: If I put "は” in the middle of " あした" and "本", is it correct too?
I would say that, while both are indeed grammatically correct, it changes the nuance somewhat. あしたは would be more equivalent to saying 'As for tomorrow, ...' . It would be a way to emphasize tomorrow rather than the action that is taking place then.
Added on 2018/05/26:
If someone asks when you will return the book, this is the proper construction.
If someone asks what you are doing tomorrow, the construction including は would be more appropriate.