[web2py] Outer join problem

[ceriox]

hi all
i have a problem making an outer join
somebody can write me the correct one?

db.define_table('Union',
    Field('user_id', db.User, writable=False, readable=False),
    Field('book_id', 'integer', db.Book, writable=False,
readable=False))

db.define_table('Book',
   Field('Titolo', 'string'))

i want to have all book with no User
i try with
record=db().select(db.Book.ALL, db.Union.ALL,
right=db.Book.on(db.Union.book_id<>db.Book.id))

but i have all Book and Book without union

what is the error?
thanks for help