flask-bootswatch: css is not loading in "/" paths

While working on a project which I build using Flask and a Bootswatch theme which is a books website, letting users to register, log in and then search for books and when a users selects one of results it gives information about that book,but I found out that information page which I implemented wasn't loading the styling of page, contrary to all other pages which are working fine. My application.py for information page is:

@app.route("/book/<isbn>")
def book(isbn):

# getting the book from database
book = db.execute("SELECT * FROM books WHERE isbn=:isbn", {"isbn": isbn}).fetchone()
if book is None:
    return render_template("failure.html", error="Please enter some query", code = "403!")
db.commit()

# giving details to webpage
return render_template("book.html", book = book)

as you can see I am using ISBN (International Standard Book Number), as a path for different books because each book has unique ISBN, thus I assume it is great for making links to different individual book page. and book.html looks something like this:

<!-- further exptending the layout page -->
{% extends "layout.html" %}

<!-- further extending the title -->
{% block title %}
  Books: Book Page
{% endblock %}

<!-- further extending the body -->
{% block body %}
  <h3 align="center"> {{book.title}} by {{book.author}}</h3>
  <div>
   <li>
     Title: {{book.title}}
   </li>
   <li>
     Author: {{book.author}}
   </li>
   <li>
     ISBN: {{book.isbn}}
   </li>
   <li>
     Year: {{book.year}}
   </li>
 </div>
  {% endblock %}

Please figure out where I am going wrong.