accepts_nested_attributes_for issue

Hello,
I am using accepts_nested_attributes_for for saving the records.
let say I have a model
class User < AR::Base
has_many :books
accepts_nested_attributes_for :books
end
class Book < AR::Base
belongs_to :user
end
Now the params value I am getting is
user={:id => 123, books_attributes={:name => “test”}}
and in my books controller
def create
user = @user.update_attributes(params[:user])
@book = ???
end
Every thing is working as expected but my requirement is that the book which is saved I need to know the ID.
How do I find the books ID.
Thanks
Abhis