kodsal hatadan dolayı diye tahmin ediyorum ama hata yok kodlarımda yardımcı olabilecek var diskord,tw gibi kodları atıyorum düzeltme yaptığım yardımcı olursanız sevinirim 

Değişiklik 1

package com.example.koko.myapplication;

import android.content.Intent;
import android.database.Cursor;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.GridView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    GridView gridView;
    ArrayList<Book> list;
    Adapter adapter=null;
    Button ekle;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ekle=findViewById(R.id.btnEkle);

         gridView=findViewById(R.id.gridView);
         list=new ArrayList<>();
         adapter=new Adapter(this,R.layout.items,list);
         gridView.setAdapter(adapter);

        Cursor cursor=Main2.sqLiteHelper.getData("SELECT*FROM BOOK");
        list.clear();
        while (cursor.moveToNext())
        {
            int id=cursor.getInt(0);
            String name=cursor.getString(1);
            String yazname=cursor.getString(2);
            String details=cursor.getString(3);
            byte[] image=cursor.getBlob(4);

            list.add(new Book(id,name,yazname,details,image));
        }
        adapter.notifyDataSetChanged();



        ekle.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) { Intent ıntent=new Intent(MainActivity.this, Main2.class);
            startActivity(ıntent);
            }
        });
    }
}

değişiklik yaptığım 2

package com.example.koko.myapplication;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;

import java.util.ArrayList;

/**
 * Created by Koko on 11.03.2018.
 */

public class Adapter extends BaseAdapter
{
    private Context context;
    private int layout;
    private ArrayList<Book> bookList;

    public Adapter(Context context, int layout, ArrayList<Book> bookList) {
        this.context = context;
        this.layout = layout;
        this.bookList = bookList;
    }

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

    @Override
    public Object getItem(int position) {
        return bookList.get(position);
    }

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

    private class ViewHolder
    {
        ImageView ımageView;
        TextView txtname,txtyazname,details;

    }

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

           View row=view;
           ViewHolder holder=new ViewHolder();

           if (row==null)
           {
               LayoutInflater ınflater=(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
               row=ınflater.inflate(layout,null);

               holder.txtname=(TextView) row.findViewById(R.id.txtkadı);
               holder.txtyazname=(TextView)row.findViewById(R.id.txtyadı);
               holder.ımageView=(ImageView) row.findViewById(R.id.listimg);
               row.setTag(holder);
           }
           else
           {
                holder=(ViewHolder) row.getTag();
           }
        Book book = bookList.get(position);

        holder.txtname.setText(book.getName());
        holder.txtyazname.setText(book.getYazname());

        byte[] bookImage = book.getImage();
        Bitmap bitmap = BitmapFactory.decodeByteArray(bookImage, 0, bookImage.length);
        holder.ımageView.setImageBitmap(bitmap);
        return row;
    }
}

fark olan yada projeye bakmak istiyende olur ödevim yapamadım yardımcı olursanız sevinirim