Follow these simple steps to add stages to the library.book.rent module:

1. Add a new model called library.rent.stage, as follows:
   

class LibraryRentStage(models.Model):
    _name = 'library.rent.stage'
    _order = 'sequence,name'

    name = fields.Char()
    sequence = fields.Integer()
    fold = fields.Boolean()
    book_state = fields.Selection(
        [('available', 'Available'),
         ('borrowed', 'Borrowed'),
         ('lost', 'Lost')],
        'State', default="available")

2. Add access rights for this new module in the security/ir.model.access.csv file, as follows:

acl_book_rent_stage,library.book_rent_stage_default,model_library_rent_stage,,1,0,0,0
acl_book_rent_librarian_stage,library.book_rent_stage_librarian,model_library_rent_stage,group_librarian,1,1,1,1

3. Remove the state field from the library.book.rent model and replace it with a new stage_id field which is a many2one field and its methods, as shown in the following example:
   

@api.model
def _default_rent_stage(self):
    Stage = self.env['library.rent.stage']
    return Stage.search([], limit=1)

stage_id = fields.Many2one(
    'library.rent.stage',
    default=_default_rent_stage
)

4. Replace the state field in the form view with the stage_id field, as shown in the following example:

<header>
    <field name="stage_id" widget="statusbar" 
          options="{'clickable': '1', 'fold_field': 'fold'}"/>
</header>

5. Replace the state field in the tree view and replace it with the stage_id field, as follows:

<tree>
    <field name="book_id"/>
    <field name="borrower_id"/>
    <field name="stage_id"/>
</tree>

6. Add some initial stages from the data/library_stage.xml file. Don't forget to add this file in the manifest, as shown in the following example:

<?xml version="1.0" encoding="utf-8"?>
<odoo noupdate="1">
    <record id="stage_draft" model="library.rent.stage">
        <field name="name">Draft</field>
        <field name="sequence">1</field>
        <field name="book_state">available</field>
    </record>
    <record id="stage_rent" model="library.rent.stage">
        <field name="name">On rent</field>
        <field name="sequence">5</field>
        <field name="book_state">borrowed</field>
    </record>
    <record id="stage_due" model="library.rent.stage">
        <field name="name">Due</field>
        <field name="sequence">15</field>
        <field name="book_state">borrowed</field>
    </record>
    <record id="stage_returned" model="library.rent.stage">
        <field name="name">Completed</field>
        <field name="sequence">25</field>
        <field name="book_state">available</field>
    </record>
    <record id="stage_lost" model="library.rent.stage">
        <field name="name">Lost</field>
        <field name="sequence">35</field>
        <field name="fold" eval="True"/>
        <field name="book_state">lost</field>
    </record>
</odoo>

After installing the module, you will see stages in the form view, as shown in the following screenshot: