Here are the frontend.py and backend.py scripts in OOP style. To execute this program you should execute the frontend.py file.

#frontend.py
from tkinter import *
from backend import Database

database=Database("books.db")

class Window(object):

    def __init__(self,window):

        self.window = window

        self.window.wm_title("BookStore")

        l1=Label(window,text="Title")
        l1.grid(row=0,column=0)

        l2=Label(window,text="Author")
        l2.grid(row=0,column=2)

        l3=Label(window,text="Year")
        l3.grid(row=1,column=0)

        l4=Label(window,text="ISBN")
        l4.grid(row=1,column=2)

        self.title_text=StringVar()
        self.e1=Entry(window,textvariable=self.title_text)
        self.e1.grid(row=0,column=1)

        self.author_text=StringVar()
        self.e2=Entry(window,textvariable=self.author_text)
        self.e2.grid(row=0,column=3)

        self.year_text=StringVar()
        self.e3=Entry(window,textvariable=self.year_text)
        self.e3.grid(row=1,column=1)

        self.isbn_text=StringVar()
        self.e4=Entry(window,textvariable=self.isbn_text)
        self.e4.grid(row=1,column=3)

        self.list1=Listbox(window, height=6,width=35)
        self.list1.grid(row=2,column=0,rowspan=6,columnspan=2)

        sb1=Scrollbar(window)
        sb1.grid(row=2,column=2,rowspan=6)

        self.list1.configure(yscrollcommand=sb1.set)
        sb1.configure(command=self.list1.yview)

        self.list1.bind('<<ListboxSelect>>',self.get_selected_row)

        b1=Button(window,text="View all", width=12,command=self.view_command)
        b1.grid(row=2,column=3)

        b2=Button(window,text="Search entry", width=12,command=self.search_command)
        b2.grid(row=3,column=3)

        b3=Button(window,text="Add entry", width=12,command=self.add_command)
        b3.grid(row=4,column=3)

        b4=Button(window,text="Update selected", width=12,command=self.update_command)
        b4.grid(row=5,column=3)

        b5=Button(window,text="Delete selected", width=12,command=self.delete_command)
        b5.grid(row=6,column=3)

        b6=Button(window,text="Close", width=12,command=window.destroy)
        b6.grid(row=7,column=3)

    def get_selected_row(self,event):
        index=self.list1.curselection()[0]
        self.selected_tuple=self.list1.get(index)
        self.e1.delete(0,END)
        self.e1.insert(END,self.selected_tuple[1])
        self.e2.delete(0,END)
        self.e2.insert(END,self.selected_tuple[2])
        self.e3.delete(0,END)
        self.e3.insert(END,self.selected_tuple[3])
        self.e4.delete(0,END)
        self.e4.insert(END,self.selected_tuple[4])

    def view_command(self):
        self.list1.delete(0,END)
        for row in database.view():
            self.list1.insert(END,row)

    def search_command(self):
        self.list1.delete(0,END)
        for row in database.search(self.title_text.get(),self.author_text.get(),self.year_text.get(),self.isbn_text.get()):
            self.list1.insert(END,row)

    def add_command(self):
        database.insert(self.title_text.get(),self.author_text.get(),self.year_text.get(),self.isbn_text.get())
        self.list1.delete(0,END)
        self.list1.insert(END,(self.title_text.get(),self.author_text.get(),self.year_text.get(),self.isbn_text.get()))

    def delete_command(self):
        database.delete(self.selected_tuple[0])

    def update_command(self):
        database.update(self.selected_tuple[0],self.title_text.get(),self.author_text.get(),self.year_text.get(),self.isbn_text.get())

window=Tk()
Window(window)
window.mainloop()

And below you will also find the backend.py script in OOP:

#backend.py
import sqlite3
class Database:
    def __init__(self, db):
        self.conn=sqlite3.connect(db)
        self.cur=self.conn.cursor()
        self.cur.execute("CREATE TABLE IF NOT EXISTS book (id INTEGER PRIMARY KEY, title text, author text, year integer, isbn integer)")
        self.conn.commit()
    def insert(self,title,author,year,isbn):
        self.cur.execute("INSERT INTO book VALUES (NULL,?,?,?,?)",(title,author,year,isbn))
        self.conn.commit()
    def view(self):
        self.cur.execute("SELECT * FROM book")
        rows=self.cur.fetchall()
        return rows
    def search(self,title="",author="",year="",isbn=""):
        self.cur.execute("SELECT * FROM book WHERE title=? OR author=? OR year=? OR isbn=?", (title,author,year,isbn))
        rows=self.cur.fetchall()
        return rows
    def delete(self,id):
        self.cur.execute("DELETE FROM book WHERE id=?",(id,))
        self.conn.commit()
    def update(self,id,title,author,year,isbn):
        self.cur.execute("UPDATE book SET title=?, author=?, year=?, isbn=? WHERE id=?",(title,author,year,isbn,id))
        self.conn.commit()
    def __del__(self):
        self.conn.close()