association_table = Table('association', Base.metadata,
    Column('book_id', Integer, ForeignKey('Book.id')),
    Column('form_id', Integer, ForeignKey('Form.id'))
    )

class Book(Base):
    __tablename__ = 'Book'
    name = Column(String(80), nullable = False, unique=True)
    id = Column(Integer, primary_key = True)
    author = Column(String(30))
    language = Column(String(30))
    category_id = Column(Integer, ForeignKey('Category.id'), nullable = False)
    category = relationship('Category')
    forms = relationship("Form", secondary=association_table, back_populates="books")

    def __repr__(self):
        return '<Book %r>' % self.name

class Form(Base):
    __tablename__ = 'Form'
    name = Column(String, nullable = False)
    id = Column(Integer, primary_key=True)
    books = relationship("Book", secondary=association_table, back_populates="forms")

    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return self.name

@app.route('/books/new/', methods=['GET', 'POST'])
def newBook():
    categories = session.query(Category).all()
    forms = session.query(Form).all()
    #languages = session.query(Languages).all()
    if request.method == 'POST':
        newBook = Book(name = request.form['name'],
                        author = request.form['author'],
                        category = session.query(Category).filter_by(name=request.form.get('category')).one(),
                        forms = [Form(name) for name in request.form.getlist('form')],
                        #works, I get both elements in the list, but I create new Form object
                        forms = [session.query(Form).filter_by(name=request.form.get('form')).one()],
                        #works, I do not create new Form object, but I get only one element in the list.
                        language = request.form['language'])
        session.add(newBook)
        session.commit()
        flash(newBook.name+'book, in category '+newBook.category.name+' added!')
        return redirect(url_for('showBooks'))
    else:
        return render_template('newBook.html', languages=languages, categories=categories, forms=forms)

<p>Choose Form</p>
    <select name=form method="GET" action="/" multiple>
    {% for form in forms %}
    <option value= "{{form}}" SELECTED>{{ form }}</option>"
    {% endfor %}
    </select>