Heroics for Beginners is a delightful light fantasy that pleases on a couple of levels.
First, it is a spoof of the conventions of the fantasy genre, almost a novelization of Peter Anspach's "Evil Overlord list". However, it also works as a light-hearted story of adventure that Joseph Campbell might recognize, for the flawed hero overcomes his shortcomings with a handy how-to book that is more effective than any magic ring.
This is a quick read; if you enjoy Terry Pratchett or Piers Anthony's lighter works, you will probably enjoy this. It is well-written enough as a story that young teens may enjoy it without really caring about all the unsubtle references to classic fantasy literature.