In order to add the post_init_hook, follow these steps:

1. Register the hook in the __manifest__.py file with the post_init_hook key:

...
'post_init_hook': 'add_book_hook',
...

2. Add the add_book_hook() method in the __init__.py file:

from odoo import api, fields, SUPERUSER_ID

def add_book_hook(cr, registry):
    env = api.Environment(cr, SUPERUSER_ID, {})
    book_data1 = {'name': 'Book 1', 'date_release': fields.Date.today()}
    book_data2 = {'name': 'Book 2', 'date_release': fields.Date.today()}
    env['library.book'].create([book_data1, book_data2])