绝对不需要三个循环，只需连接数据并循环一次

begin

for i in (select b.bookid,
                 b.title,
                 p.publishername as publisher,
                 a.firstname || ' ' || a.lastname as author
            from books b
      inner join publisher p on b.publisherid = p.publisherid 
      inner join author a on b.author_id = a.authorid) loop

      dbms_output.put_line(i.bookid || ' ... ' || i.title || ' ... ' || i.publisher || ' ... ' || i.author);
end loop;

end;