Here are the changes I made to do the Pre Assessment Excercise.

addons_library/library_extensions/__init__.py

@@ -0,0 +1,3 @@
+# -*- coding: utf-8 -*-
+
+from . import models

addons_library/library_extensions/__manifest__.py

@@ -0,0 +1,27 @@
+# -*- coding: utf-8 -*-
+{
+    "name": "Library Extensions", #Changed from Library
+    "version": "17.0.1.0",
+    "depends": [
+        "base",
+        "library"
+
+    ],
+    "external_dependencies": {},
+    "author": "Odev Solutions",
+    "website": "https://www.odevsolutions.com",
+    "summary": """Library""",
+    "description": """
+        Library
+    """,
+    "category": "Extra Tools",
+    "data": [
+        "security/ir.model.access.csv",
+        "views/library_book_category_views.xml", # Added
+        "views/library_book_views.xml",
+        "views/library_menus.xml",
+
+    ],
+    "installable": True,
+    "application": False,
+}

addons_library/library_extensions/models/__init__.py

@@ -0,0 +1,4 @@
+# -*- coding: utf-8 -*-
+
+from . import library_book
+from . import library_book_category

addons_library/library_extensions/models/library_book.py

@@ -0,0 +1,44 @@
+# -*- coding: utf-8 -*-
+
+from odoo import models, fields, api
+
+
+class LibraryBook(models.Model):
+    _inherit = "library.book"
+
+    # Task 2: Added Author field
+    author_id = fields.Many2one(
+        'res.partner', 
+        string="Author", 
+        required=True
+    )
+
+    # Task 3: Added Category field
+    category_ids = fields.Many2many(
+        'library.book.category', 
+        string="Categories"
+    )
+
+    ######################
+    # Fields declaration #
+    ######################
+
+    ##############################
+    # Compute and search methods #
+    ##############################
+
+    ############################
+    # Constrains and onchanges #
+    ############################
+
+    #########################
+    # CRUD method overrides #
+    #########################
+
+    ##################
+    # Action methods #
+    ##################
+
+    ####################
+    # Business methods #
+    ####################

addons_library/library_extensions/models/library_book_category.py

@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*-
+import random  # <--- #Newly Added Import
+from odoo import models, fields
+
+class LibraryBookCategory(models.Model):
+    _name = "library.book.category"
+    _description = "Book Category"
+
+    name = fields.Char(string="Name", required=True)
+    # Added to fix error when adding category for adding a book entry.
+    color = fields.Integer(
+        string="Color Index",
+        default=lambda self: random.randint(1, 11)
+        )
+
+    _sql_constraints = [
+        ('name_unique', 'unique(name)', 'The category name must be unique!')
+    ]

addons_library/library_extensions/security/ir.model.access.csv

@@ -0,0 +1,3 @@
+id,name,model_id/id,group_id/id,perm_read,perm_write,perm_create,perm_unlink
+access_library_book_admin,access.library.book.admin,model_library_book,base.group_system,1,1,1,1
+access_library_book_category_admin,access.library.book.category.admin,model_library_book_category,base.group_system,1,1,1,1

addons_library/library_extensions/views/library_book_category_views.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?>
+<odoo>
+    <record id="library_book_category_view_tree" model="ir.ui.view">
+        <field name="name">library.book.category.view.tree</field>
+        <field name="model">library.book.category</field>
+        <field name="arch" type="xml">
+            <tree string="Categories" editable="bottom">
+                <field name="name"/>
+            </tree>
+        </field>
+    </record>
+
+    <record id="library_book_category_action" model="ir.actions.act_window">
+        <field name="name">Book Categories</field>
+        <field name="res_model">library.book.category</field>
+        <field name="view_mode">tree,form</field>
+    </record>
+</odoo>

addons_library/library_extensions/views/library_book_views.xml

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="utf-8"?>
+<odoo>
+    <record id="library_book_view_tree_inherit" model="ir.ui.view">
+        <field name="name">library.book.view.tree.inherit</field>
+        <field name="model">library.book</field>
+        <field name="inherit_id" ref="library.library_book_view_tree"/>
+        <field name="arch" type="xml">
+            <xpath expr="//field[@name='isbn_13']" position="after">
+                <field name="author_id"/>
+                <field name="category_ids" widget="many2many_tags" options="{'color_field': 'color'}"/>
+            </xpath>
+        </field>
+    </record>
+
+    <record id="library_book_view_form_inherit" model="ir.ui.view">
+        <field name="name">library.book.view.form.inherit</field>
+        <field name="model">library.book</field>
+        <field name="inherit_id" ref="library.library_book_view_form"/>
+        <field name="arch" type="xml">
+            <xpath expr="//field[@name='isbn_13']" position="after">
+                <field name="author_id"/>
+                <field name="category_ids" widget="many2many_tags" options="{'color_field': 'color'}"/>
+            </xpath>
+        </field>
+    </record>
+</odoo>

addons_library/library_extensions/views/library_menus.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<odoo>
+    <menuitem
+        id="library_book_category_menu"
+        name="Book Categories"
+        parent="library.library_menu_main"
+        action="library_book_category_action"
+        sequence="20"/>
+</odoo>

config/odoo.conf

@@ -2,4 +2,5 @@
 addons_path = /mnt/addons_library
 data_dir = /var/lib/odoo
 proxy_mode = True
+admin_passwd = $pbkdf2-sha512$600000$qzVGiBFCKGXs/d8bw7hXSg$UGyJ64Y/mFFkfgR3anPkDmOeX5CSoOHSogy5dEEnUz96zpW5q34gTPr21/SAka/5XFPpTlfpdo0E9VWqXjjA4w