procedure TForm1.SaveClick(Sender: TObject);
const
  MyXML = 'C:\MyFile\Book.xml';
var
  XML: TJclSimpleXML;
  Root: TJclSimpleXMLElem;
  I: Integer;
  Section, DecStr: string;
  Str: TStringList;
begin
  DecStr := combobox1.Text; // Section selected of combobox
  if FileExists(MyXML) then
  begin
    XML := TJclSimpleXML.Create;
    Str := TStringList.Create;
    try
      XML.LoadFromFile(MyXML);
      XML.Root.Name := 'library';
      Root := XML.Root;
      for I := 0 to XML.Root.ItemCount - 1 do
      begin
        Section := Root.Items.Item[I].Properties.Value('name');
        if SameText(Section, DecStr) = True then
        begin
          begin
            with XML.Root.Items.Item[I].Items.Insert('IDX', combobox1.ItemIndex) do
            begin
              Properties.Add('author', edit1.Text);
              Properties.Add('book', EntityEncode(edit2.Text));
            end;
          end;
        end;
      end;
      DecStr := XMLDecode(XML.SaveToString);
      Str.Add(DecStr);
      Str.SaveToFile(MyXML , TEncoding.Default);
    finally
      XML.Free;
      FreeAndNil(Str);
    end;
  end;
end;