"You write a book."
Translation:Tu écris un livre.
I am guessing that you're asking about the difference between "Tu écris un livre" and "Vous écrivez un livre"?
The first is talking to a single person, the second is formally talking to someone (which English doesn't have an equivalent for), or talking to multiple people at the same time. "tu" is the 2nd person singular pronoun for informal speech, and "vous" is the 2nd person plural pronoun, or 2nd person singular pronoun for formal speech. All of those pronouns are just "you" in English", so there is some ambiguity when reading the English sentence.