function and passing the table name to be used with SQL stmnt

ok so my probelm is I have a simple function like this
CREATE OR REPLACE FUNCTION bookid(_tabela varchar) RETURNS integer AS $
$
DECLARE
    _id integer := 0;
BEGIN
    SELECT INTO _id id FROM _tabela order by id desc limit 1;

    RETURN _id+1;
END;
$$ LANGUAGE plpgsql;

and the problem is i can run this function cos im passing string to
SQL statement and not the table object. hym what can I do about it to
have function that will give me next id number back;