based on gdv cell selecion value get into textbox

Try something like this:

private void dataGridView1_CellDoubleClick(Object sender, DataGridViewCellEventArgs e) 
{
   if(e.ColumnIndex == 0) // book_id column
   {
      object val = dataGridView1.Rows[e.RowIndex].Cells[0].Value;
      if (val != null)
      {
         string connString = "whatever";

         using (SqlConnection conn = new SqlConnection(connString))
         {
            conn.Open();

            using (SqlCommand cmd = new SqlCommand("SELECT quantity_available FROM book_stock WHERE book_id = @book_id", conn))
            {
                cmd.Parameters.Add(new SqlParameter("@book_id", val.ToString())); // assuming book_id column is character based
                SqlDataReader reader = cmd.ExecuteReader();
                if(reader.Read()) textBox1.Text = reader[0].ToString();
                reader.Close();
            }
           
            conn.Close();
         }
      }
   }
}

Hi,

Use row command get the value of bookId and by passing bookid you can get quantity.

protected void GridView1_RowCommand(object sender, System.Web.UI.WebControls.GridViewCommandEventArgs e)
   {
        if (e.CommandName=="MyCommand")
        {   
             int row = int.Parse(e.CommandArgument.ToString());
             var cellText= gvOwner.Rows[row].Cells["bookID"].Text.Trim();
        }
   }

Hope this will help.