public class Book
{


        public virtual long ID { get;  set;  }

        public virtual string ISBN { get; set; }

        public virtual string Title { get; set; }

        public virtual Theme Theme { get; set; }

		public Book()
		{

		}

        public Book(string isbn, string title, long id_theme)
		{
            this.Theme = new Theme(id_theme);
            ISBN = isbn;
            Title = title;
            
		}

        public virtual void AddAutor(string name)
        {
            Autor a = new Autor(name);
            if (Autors == null)
                Autors = new HashSet<Autor>();
            Autors.Add(a);
        }

        public virtual ICollection<Autor> Autors
        {
            get;
            set;
        }
}

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="NHibernate_1" assembly="NHibernate_1">
  <class name="Book" table="Books">
    <id name="ID" unsaved-value="0">
      <column name="ID" not-null="true" />
        <generator class="identity"/>
    </id>
    
    <property name="ISBN" />
    <property name="Title" />


    <many-to-one  class="Theme" name="Theme" column="Id_Theme" />

    <set cascade="save-update" lazy="true" name="Autors" table="Autor">
      <key column="id_book" />
      <one-to-many class="Autor" />
    </set>
  </class>
</hibernate-mapping>

Book book = new Book("ISBN", "Новое название книги", 1);
                book.AddAutor("Автор1");
                book.AddAutor("Автор2");
                book.AddAutor("Автор3");
                session.Save(book);

<many-to-one name="Book" column="id_book"  class="Book"  />

public virtual void AddAutor(string name)
        {
            Autor a = new Autor(name);
            if (Autors == null)
                Autors = new HashSet<Autor>();
            a.Book = this;
            Autors.Add(a);
        }