Adapter always shows me the same item in ListView

I am developing an Android application that uses Google Sheets as a database. The operation, basically, is that I scan a bar code from a book, it keeps all the information about this book in the sheet of Google Sheets, and what I want is that this information shows it to me in ListView .

When I scan two books, I keep them correctly on the sheet, but when I want to show this information in the ListView , it shows me the last one I inserted, and I do not understand why.

Next I am showing the code and explaining what it returns to me:

The following method reads the information from the spreadsheet and inserts it into the List<Strings>results :

private List<String> getDataFromApi() throws IOException {

        String range = "Sheet1!A2:I";
        List<String> results = new ArrayList<String>();
        ValueRange response = CreateSpreadsheets.mService.spreadsheets().values()
                //.get(CreateSpreadsheets.spreadsheet_id, range)
                .get(sheet_id, range)
                .execute();
        List<List<Object>> values = response.getValues();
        System.out.println(values);
        if (values != null) {
            for (List row : values) {
                results.add(row.get(0) + "," + row.get(6));
            }
        }
        return results;
    }

Next, the postExecute

@Override
    protected void onPostExecute(List<String> results) {
        mProgress.hide();
        if (results == null || results.size() == 0) {
           // emptyText.setText(R.string.empty);
        } else {
            if(read_only.equals("no")) {
                Intent intent = new Intent(Spreadsheets.this, MainActivity.class);
                startActivity(intent);
                //  mOutputText.setText("Se ha añadido un libro a su lista");
            }
            else {
                resultados = new ArrayList<BookItem>();
                System.out.println(results);
                int length = results.size();
                System.out.println(length);
                int i;
                for (i = 0; i < length; i++) {
                    String string = results.get(i);
                    System.out.println("string results get (i)" + string);
                    String[] partes = string.split(",");
                    String titulo = partes[0];
                    String portada = partes[1];
                    System.out.println(titulo + "," + portada);
                    book = new BookItem(titulo, portada);
                    resultados.add(book);
                }
                read_only = "no";
                rellenar();
            }
        }
    }

Here I have put a couple of prints to check that it is really giving me back all the books I have saved, and indeed, that's the way it is:

I/System.out: string results get (i)Todo esto te daré,No portada
I/System.out: Todo esto te daré,No portada
I/System.out: string results get (i)La elegancia del erizo,http://books.google.com/books/content?id=od5GRQAACAAJ&printsec=frontcover&img=1&zoom=1&source=gbs_api
I/System.out: La elegancia del erizo,http://books.google.com/books/content?id=od5GRQAACAAJ&printsec=frontcover&img=1&zoom=1&source=gbs_api

Next, the rellenar() function does the following:

public void rellenar(){
        adapter = new BookAdapter(context, resultados, sheet_id);
        bookList.setAdapter(adapter);
}

Then I leave the code of my BookAdapter :

public class BookAdapter extends BaseAdapter {

List<BookItem> items;
private Context context;
LayoutInflater inflater;


public BookAdapter(Context context, List<BookItem> items) {
    this.context = context;
    this.items = items;
    inflater = LayoutInflater.from(context);
}

@Override
public int getCount() {
    return items.size();
}

@Override
public BookItem getItem(int position) {
    return this.items.get(position);
}

@Override
public long getItemId(int i) {
    return i;
}

@Override
public View getView(final int position, View convertView, ViewGroup viewGroup) {

    ViewHolder holder;

    if(convertView == null){
        convertView = inflater.inflate(R.layout.fila_lista_miestanteria, viewGroup, false);
        holder = new ViewHolder();
        holder.title_item = (TextView) convertView.findViewById(R.id.book_title_item);
        holder.favorite = (ImageButton) convertView.findViewById(R.id.favorite);
        holder.cover_item = (ImageView) convertView.findViewById(R.id.book_cover_item);

        holder.title_item.setText(this.items.get(position).getTitle());
        convertView.setTag(holder);
    }
    else {
        holder = (ViewHolder) convertView.getTag();
    }
    BookItem bookItem = getItem(position);
    System.out.println("position en bookdapter " + getItem(position));
    holder.title_item.setText(bookItem.getTitle());
    System.out.println("Titulo: " + bookItem.getTitle());
    holder.favorite.setImageResource(R.drawable.ic_favorite_black_24dp);
    final String url = bookItem.getCover();
    Picasso.with(context).load(url).into(holder.cover_item);

    return convertView; }

The prints that I have put (with the for, now commented) it seems that they are giving me back two different books:

I/System.out: size 2
I/System.out: position com.example.apptfg.BookItem@aba768a
I/System.out: 2
I/System.out: position com.example.apptfg.BookItem@aba768a
I/System.out: size 2
I/System.out: position com.example.apptfg.BookItem@d17156
I/System.out: 2
I/System.out: position com.example.apptfg.BookItem@d17156

But finally the result in my ListView is the last book I have inserted, and not the two that I have in the list:

Could someone help me? Thank you very much in advance!