what is the problem?

private static void InsertSomeData()
{
   // http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnectionstringbuilder.aspx
   // create proper connection string
   SqlConnectionStringBuilder sb = new SqlConnectionStringBuilder();
   sb.DataSource = @"your server";
   sb.InitialCatalog = @"your db";
   sb.IntegratedSecurity = true;

   // sql insert statement
   string cmdStr = "INSERT INTO Data (id_book,id_person,name_book) VALUES (@id_book, @id_person, @name_book)";

   // http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection%28v=vs.90%29.aspx
   // http://msdn.microsoft.com/en-us/library/vstudio/system.idisposable%28v=vs.100%29.aspx

   // use SqlConnection with using statement
   using (SqlConnection conn = new SqlConnection(sb.ConnectionString))
   {
      // open connection
      conn.Open();
      // http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand%28v=vs.90%29.ASPX

      using (SqlCommand cmd = new SqlCommand(cmdStr, conn))
      {
         // use SqlCommand.Parameters to add parameters
         cmd.Parameters.AddWithValue("@id_book", "your value");
         cmd.Parameters.AddWithValue("@id_person", "your value");
         cmd.Parameters.AddWithValue("@name_book", "your value");

         // http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.executenonquery%28v=vs.90%29.ASPX
         // insert with ExecuteNonQuery since insert statement is not a query
         int insertCount = cmd.ExecuteNonQuery();
      }
   }
}

CREATE TABLE [dbo].[TABLE_A](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [NAME] [varchar](100) NOT NULL,
PRIMARY KEY CLUSTERED
(
    [ID] ASC
)

using System;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string connStr = @"data source=.\sqlexpress; initial catalog=TEST; integrated security=true;";
            string cmdStr = @"insert into TABLE_A(NAME) values(@name)";
            using (SqlConnection conn = new SqlConnection(connStr))
            {
                conn.Open();
                using (SqlCommand cmd = new SqlCommand(cmdStr, conn))
                {
                    cmd.Parameters.AddWithValue("@name", "my name");
                    dynamic o = cmd.ExecuteScalar();
                    Console.WriteLine(o == null ? "null" : o);

                    o = cmd.ExecuteNonQuery();
                    Console.WriteLine(o == null ? "null" : o);
                }
            }
        }
    }
}

Quote:

null
1
Press any key to continue . . .

SqlCommand add = new SqlCommand("INSERT INTO Data (id_book,id_person,name_book) VALUES (" + textBox7.Text + "," + textBox8.Text + ",'" + textBox9.Text + "')",con);