AddAuthor.java

Book b = em.find(Book.class, 1L);

Author a = new Author();
a.setFirstName("Thorben");
a.setLastName("Janssen");

a.getBooks().add(b);
b.getAuthors().add(a);

em.persist(a);

Author.java

@Entity
public class Author {

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	@Column(name = "id", updatable = false, nullable = false)
	private Long id;

	@ManyToMany(mappedBy="authors")
	private List<Book> books = new ArrayList<Book>();

	...
}

Book.java

@Entity
public class Book {

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	@Column(name = "id", updatable = false, nullable = false)
	private Long id;

	@ManyToMany
	@JoinTable(name = "book_author",
		joinColumns = { @JoinColumn(name = "fk_book") },
		inverseJoinColumns = { @JoinColumn(name = "fk_author") })
	private List<Author> authors = new ArrayList<Author>();

	...
}

FindBook.java

b = em.find(Book.class, 1L);
List<Author> authors = b.getAuthors();

HelperMethods.java

@Entity
public class Author {

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	@Column(name = "id", updatable = false, nullable = false)
	private Long id;

	@ManyToMany(mappedBy="authors")
	private List<Book> books = new ArrayList<Book>();

	public void addBook(Book b) {
		this.books.add(b);
		b.getAuthors().add(this);
	}

	public void removeBook(Book b) {
		this.books.remove(b);
		b.getAuthors().remove(this);
	}

	...
}