What does this () alone do in the constructor?

It invokes the constructor that does not receive parameters, for example in the expression Livro l = new Livro(autor); what will occur is as follows:

You could rewrite the code as equivalent:

public Livro(Autor autor) { 
    this.isbn = "000-00-00000-00-0"; // ao invés de invocar o construtor Livro() estamos ralizando o que o tal construtor faz.
    this.autor = autor;
}
public Livro() { 
    this.isbn = "000-00-00000-00-0";
}

this is a reference to the current object - the object whose method or constructor is being called to.

When used inside one constructor it is used to call another constructor in the same class.

Your Book class declares two constructors:

public Livro(Autor autor)

and

public Livro()

In the public Livro() constructor the isbn attribute is initialized, which must (I assume) be initialized regardless of the constructor used, hence the need to be called by the public Livro(Autor autor) constructor.

I think the code you posted is just an example, but here is a "more correct" way to declare constructors:

public class Livro{

    private String isbn;
    private Autor autor; 

    public Livro(Autor autor, String isbn) { 
        this.autor = autor;
        this.isbn = isbn;
    }

    //Talvez passar ""(empty) na vez de "000-00-00000-00-0"
    public Livro(Autor autor) { 
        this(autor, "000-00-00000-00-0");
    }

    //Talvez não se devesse criar um livro sem autor.
    //Ou talvez sim, se o autor for desconhecido.
    public Livro() { 
        //Em vez de null talvez fosse preferível criar um Autor "default"(desconhecido)
        this(null);
    }
}