Book.java

@Entity
public class Book {

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

	@Column
	private String title;

	@OneToMany(mappedBy = "book")
	private Set<BookPublisher> publishers = new HashSet<BookPublisher>();
	
	...
	
}

BookPublisher.java

@Entity
public class BookPublisher {
  
	@EmbeddedId
	private BookPublisherId id;

	@ManyToOne
	@JoinColumn(name = "fk_book", insertable = false, updatable = false)
	private Book book;

	@ManyToOne
	@JoinColumn(name = "fk_publisher", insertable = false, updatable = false)
	private Publisher publisher;

	@Column
	@Enumerated(EnumType.STRING)
	private Format format;

	public BookPublisher(Book b, Publisher p, Format f) {
		// create primary key
		this.id = new BookPublisherId(b.getId(), p.getId());
		
		// initialize attributes
		this.book = b;
		this.publisher = p;
		this.format = f;
		
		// update relationships to assure referential integrity
		p.getBooks().add(this);
		b.getPublishers().add(this);
	}
	
	...
	
}

BookPublisherId.java

@Entity
public class BookPublisher {

	@Embeddable
	public static class BookPublisherId implements Serializable {

		@Column(name = "fk_book")
		protected Long bookId;

		@Column(name = "fk_publisher")
		protected Long publisherId;

		public BookPublisherId() {
			
		}
		
		public BookPublisherId(Long bookId, Long publisherId) {
			this.bookId = bookId;
			this.publisherId = publisherId;
		}

		@Override
		public int hashCode() {
			final int prime = 31;
			int result = 1;
			result = prime * result
					+ ((bookId == null) ? 0 : bookId.hashCode());
			result = prime * result
					+ ((publisherId == null) ? 0 : publisherId.hashCode());
			return result;
		}

		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			
			BookPublisherId other = (BookPublisherId) obj;
			
			if (bookId == null) {
				if (other.bookId != null)
					return false;
			} else if (!bookId.equals(other.bookId))
				return false;
			
			if (publisherId == null) {
				if (other.publisherId != null)
					return false;
			} else if (!publisherId.equals(other.publisherId))
				return false;
			
			return true;
		}
	}
	
	...
	
}

CreateBookPublisher.java

EntityManager em = emf.createEntityManager();
em.getTransaction().begin();

Book b = new Book();
b.setTitle("My Book");
em.persist(b);

Publisher p = new Publisher();
p.setName("My Publisher");
em.persist(p);

BookPublisher bp = new BookPublisher(b, p, Format.EBOOK);
em.persist(bp);

em.getTransaction().commit();
em.close();

Publisher.java

@Entity
public class Publisher {

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

	@Column
	private String name;

	@OneToMany(mappedBy = "publisher")
	private Set<BookPublisher> books = new HashSet<BookPublisher>();
	
	...
	
}