The Secret Series by Psuedonymous Bosch

This is an interesting series about a girl and her two friends who become a part of a mysterious society that helps to protect an ancient secret from the clutches of another evil secret society who wants to know the secret. Sounds very secretive, right? It is by the infamous chocolate lover, Psuedonymous Bosch. The books in the series are 'The Name Of This Book Is Secret', 'If You're Reading This, It's Too Late', 'This Book Is Not Good For You', 'This Isn't What It Looks Like', and 'You Have To Stop This'. All are very good books. They are best when read by children ages 10-13.