diff --git a/app/database.py b/app/database.py
index 827e974..93a25b5 100644
--- a/app/database.py
+++ b/app/database.py
@@ -2,15 +2,17 @@ from sqlmodel import create_engine, SQLModel, Session
 from dotenv import load_dotenv
 import os
 
+# Get database configuration
 load_dotenv()
-
 DATABASE_URL = os.getenv("DATABASE_URL")
 if not DATABASE_URL:
     raise ValueError("DATABASE_URL environment variable is not set")
 
+# Create database engine
 engine = create_engine(DATABASE_URL, echo=True)
 SQLModel.metadata.create_all(engine)
 
+# Get database session
 def get_session():
     with Session(engine) as session:
         yield session
diff --git a/app/main.py b/app/main.py
index 402b1e0..f45fe76 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1,19 +1,23 @@
-from fastapi import FastAPI
+from fastapi import FastAPI, HTTPException
 from sqlmodel import SQLModel, Session, select
+from typing import List
 from .database import engine
 from .models import Author, Book
 
 app = FastAPI()
 
+# Initialize the database
 @app.on_event("startup")
 def on_startup():
     SQLModel.metadata.create_all(engine)
 
+# Root endpoint
 @app.get("/")
-async def read_root():
-    return {"message": "Hello, FastAPI with SQLModel and PostgreSQL!"}
+async def hello_world():
+    return {"message": "Hello world!"}
 
-@app.post("/authors/")
+# Create an author
+@app.post("/authors/", response_model=Author)
 def create_author(author: Author):
     with Session(engine) as session:
         session.add(author)
@@ -21,13 +25,39 @@ def create_author(author: Author):
         session.refresh(author)
         return author
 
-@app.get("/authors/")
+# Read authors
+@app.get("/authors/", response_model=List[Author])
 def read_authors():
     with Session(engine) as session:
         authors = session.exec(select(Author)).all()
         return authors
 
-@app.post("/books/")
+# Update an author
+@app.put("/authors/{author_id}", response_model=Author)
+def update_author(author_id: int, author: Author):
+    with Session(engine) as session:
+        db_author = session.get(Author, author_id)
+        if not db_author:
+            raise HTTPException(status_code=404, detail="Author not found")
+        db_author.name = author.name
+        session.add(db_author)
+        session.commit()
+        session.refresh(db_author)
+        return db_author
+
+# Delete an author
+@app.delete("/authors/{author_id}")
+def delete_author(author_id: int):
+    with Session(engine) as session:
+        db_author = session.get(Author, author_id)
+        if not db_author:
+            raise HTTPException(status_code=404, detail="Author not found")
+        session.delete(db_author)
+        session.commit()
+        return {"message": "Author deleted"}
+
+# Create a book
+@app.post("/books/", response_model=Book)
 def create_book(book: Book):
     with Session(engine) as session:
         session.add(book)
@@ -35,8 +65,34 @@ def create_book(book: Book):
         session.refresh(book)
         return book
 
-@app.get("/books/")
+# Read books
+@app.get("/books/", response_model=List[Book])
 def read_books():
     with Session(engine) as session:
         books = session.exec(select(Book)).all()
         return books
+
+# Update a book
+@app.put("/books/{book_id}", response_model=Book)
+def update_book(book_id: int, book: Book):
+    with Session(engine) as session:
+        db_book = session.get(Book, book_id)
+        if not db_book:
+            raise HTTPException(status_code=404, detail="Book not found")
+        db_book.title = book.title
+        db_book.authors = book.authors
+        session.add(db_book)
+        session.commit()
+        session.refresh(db_book)
+        return db_book
+
+# Delete a book
+@app.delete("/books/{book_id}")
+def delete_book(book_id: int):
+    with Session(engine) as session:
+        db_book = session.get(Book, book_id)
+        if not db_book:
+            raise HTTPException(status_code=404, detail="Book not found")
+        session.delete(db_book)
+        session.commit()
+        return {"message": "Book deleted"}
diff --git a/app/models.py b/app/models.py
index 2257560..7405afe 100644
--- a/app/models.py
+++ b/app/models.py
@@ -1,16 +1,19 @@
 from typing import Optional, List
 from sqlmodel import SQLModel, Field, Relationship
 
+# Relationship model
 class AuthorBookLink(SQLModel, table=True):
     author_id: int | None = Field(default=None, foreign_key="author.id", primary_key=True)
     book_id: int | None = Field(default=None, foreign_key="book.id", primary_key=True)
 
+# Author model
 class Author(SQLModel, table=True):
     id: Optional[int] = Field(primary_key=True, index=True)
     name: str
 
     books: List["Book"] = Relationship(back_populates="authors", link_model=AuthorBookLink)
 
+# Book model
 class Book(SQLModel, table=True):
     id: Optional[int] = Field(primary_key=True, index=True)
     title: str