This recipe assumes that you have an instance ready, with the my_module addon module available, as described in Chapter 4, Creating Odoo Addon Modules. You will need to add a state field to the LibraryBook model, defined as follows:

from odoo import models, fields, api 
class LibraryBook(models.Model): 
    # [...] 
    state = fields.Selection([('draft', 'Unavailable'), 
                              ('available', 'Available'), 
                              ('borrowed', 'Borrowed'), 
                              ('lost', 'Lost')], 
                             'State') 

Refer to the recipe Adding models in Chapter 4, Creating Odoo Addon Modules for more clarity.