CREATE TABLE [dbo].[BOOKS](
	[ID] [int] IDENTITY( 1 , 1 ) NOT NULL,
	[ISBN] [nvarchar]( 255 ) NULL,
	[TITLE] [nvarchar]( 255 ) NULL,
 CONSTRAINT [PK_BOOKS] PRIMARY KEY CLUSTERED 
(
	[ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

public class Book
{
    public class Properties
    {
        public const string Book = "Book";
        public const string ID = "ID";
        public const string ISBN = "ISBN";
        public const string TITLE = "TITLE";

        private Properties() { }
    }

    public Book()
    {
    }

    public Book(string isbn, string title)
    {
        ISBN = isbn;
        TITLE = title;
    }

    public virtual int ID { get; set; }
    public virtual string ISBN { get; set; }
    public virtual string TITLE { get; set; }
}

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="MySpace" assembly="MySpace">
	<class name="Book" table="BOOKS">
		<id name="ID">
			<column name="ID" />
			<generator class="MySpace.MyGenerator, MySpace" />
		</id>
		<property name="ISBN" />
		<property name="TITLE" />
	</class>
</hibernate-mapping>

public class MyGenerator : IIdentifierGenerator 
{
    public object Generate(ISessionImplementor session, object obj)
    {
        return 3;
    }
}