This recipe assumes that you have an instance ready, with the my_library add-on module available, as described in Chapter 4, Creating Odoo Add-On Modules. You will need to add a state field to the LibraryBook model, which is 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', default="draft")

Refer to the Adding models recipe in Chapter 4, Creating Odoo Add-On Modules, for more information.