@Entity
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String title;
    private String isbn;
    private String publisher;

    @ManyToMany
    @JoinTable(name = "author_book", joinColumns = @JoinColumn(name = "book_id"),
    inverseJoinColumns = @JoinColumn(name = "author_id"))
    private Set<Author> authors = new HashSet<>();
}

@Entity
public class Author {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String firstName;
    private String lastName;

    @ManyToMany(mappedBy = "authors")
    private Set<Book> books  = new HashSet<>();
}