Translation:I will return the book to the library tomorrow.
あした is a time expression, not an adjective. So it does not modify 本. And since it is a relative time expression, it does not require a particle, like に, to define its role in the sentence. It can be used without anything separating it from the next word.
Sometimes, these kind of time expressions are followed by a comma to help the reader correctly parse the sentence. But it is not required.
If you wanted to say "Tomorrow's book" in Japanese, you would want to use the particle の to define the desired grammatical relationship between these words.