In English, these two are different concepts:
You don't have to read the book.
You must not read the book.
But in German, I believe the same sentence would work for either one.
Du musst das Buch nicht lesen.
Reading this sentence in isolation, I would suppose it meant "You don't have to read the book." Is there a way to force an interpretation of "You must not read the book."? Would you have to use dürfen for that? Maybe like:
Du darfst das Buch nicht lesen.