Foreing key Book x Authors

Olá eu comprei o livro JAVA EE do Alberto Sousa e na parte de associação Book x Author fazemos um select dos autores para salvar um book. Mas ao invés de no html ter uma lista de autores para escolher eu gostaria de o usuario pudesse inserir o codigo do autor (que já esta em uma tabela author) eu conseguisse verificar se existe e salvar o book. Como posso conseguir incluir o autor correto no book? Segue o código que estou em duvida:

HTML

<div>
            <h:outputLabel>Autores</h:outputLabel>
            <h:selectManyListbox value="#{adminBooksBean.selectedAuthorsIds}">
                <f:selectItems value="#{adminBooksBean.authors}" var="author"
                    itemLabel="#{author.name}" itemValue="#{author.id}" />

            </h:selectManyListbox>
        </div>
        <h:commandButton value="Gravar" action="#{adminBooksBean.save}" />

public class AuthorDAO {

    @PersistenceContext
    private EntityManager manager;

    public List<Author> list() {
        return manager.createQuery(
                "select a from Author a order by a.name asc", Author.class)
                .getResultList();
    }

}

@Model
public class AdminBooksBean {

    private Book product = new Book();
    @Inject
    private BookDAO productDAO;
    @Inject
    private AuthorDAO authorDAO;
    private List<Author> authors = new ArrayList<Author>();
    private List<String> selectedAuthorsIds = new ArrayList<>();

    @Transactional
    public void save(){
        populateBookAuthor();        
        productDAO.save(product);
    }

    private void populateBookAuthor() {
        selectedAuthorsIds.stream().map( (strId) -> {
            return new Author(Integer.parseInt(strId));
        }).forEach(product :: add);
    }

    public void setSelectedAuthorsIds(List<String> selectedAuthorsIds) {
        this.selectedAuthorsIds = selectedAuthorsIds;
    }

    public List<String> getSelectedAuthorsIds() {
        return selectedAuthorsIds;
    }

    public Book getProduct() {
        return product;
    }

    public List<Author> getAuthors() {
        return authors;
    }

    @PostConstruct
    private void loadObjects(){
        this.authors = authorDAO.list();
    }

}

@Entity
public class Author {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String name;

    /**
     * Hibernate only
     */
    private Author(){

    }

    public Author(Integer id) {
        this.id = id;
    }

@Entity
public class Book {

    @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String title;
    private String description;
    private int numberOfPages;
    private BigDecimal price;
    @ManyToMany
    private List<Author> authors = new ArrayList<>();