In order to create custom settings options, follow these steps:

1. Add a new group in the my_library/security/groups.xml file:

    <record id="group_self_borrow" model="res.groups">
        <field name="name">Self borrow</field>
        <field name="users" eval="[(4, ref('base.user_admin'))]"/>
    </record>

2. Add a new field by inheriting the res.config.settings model:

class ResConfigSettings(models.TransientModel):
    _inherit = 'res.config.settings'

    group_self_borrow = fields.Boolean(string="Self borrow",
                                       implied_group='my_library.group_self_borrow')

3. Add this field in the existing settings view with xpath (for more details, refer to Chapter 10, Backend Views):

<record id="res_config_settings_view_form" model="ir.ui.view">
    <field name="name">res.config.settings.view.form.inherit.library</field>
    <field name="model">res.config.settings</field>
    <field name="priority" eval="5"/>
    <field name="inherit_id" ref="base.res_config_settings_view_form"/>
    <field name="arch" type="xml">
        <xpath expr="//div[hasclass('settings')]" position="inside">
            <div class="app_settings_block" 
                 data-string="Library" string="Library" data-key="my_library" 
                 groups="my_library.group_librarian">
                <h2>Library</h2>
                <div class="row mt16 o_settings_container">
                    <div class="col-12 col-lg-6 o_setting_box" id="library">
                        <div class="o_setting_left_pane">
                            <field name="group_self_borrow"/>
                        </div>
                        <div class="o_setting_right_pane">
                            <label for="group_self_borrow"/>
                            <div class="text-muted">
                                Allow users to borrow and return books by themself
                            </div>
                        </div>
                    </div>
                </div>
            </div>
        </xpath>
    </field>
</record>

4. Add some actions and a menu for the Settings:

<record id="library_config_settings_action" model="ir.actions.act_window">
    <field name="name">Settings</field>
    <field name="type">ir.actions.act_window</field>
    <field name="res_model">res.config.settings</field>
    <field name="view_id" ref="res_config_settings_view_form"/>
    <field name="view_mode">form</field>
    <field name="target">inline</field>
    <field name="context">{'module' : 'my_library'}</field>
</record>

<menuitem name="Settings" 
    id="library_book_setting_menu"
    parent="library_base_menu"
    action="library_config_settings_action"
    sequence="50"/>

5. Modify the buttons in the book's form view and add a my_library.group_self_borrow group:

<button name="book_rent" 
    string="Rent this book" 
    type="object" class="btn-primary" 
    groups="my_library.group_self_borrow"/>
<button
    name="return_all_books"
    string="Return all book"
    type="object" class="btn-primary"
    groups="my_library.group_self_borrow"/>

Restart the server and update the my_library module to apply the changes.