Destroy after reading: the world of secret codes by Mary Colson

The black book of buried secrets by [written by Mallory Kass] ; introduction by Rick Riordan

Betrayed! 1977: the journal of Zeke Moorie by Bill Doyle

The secret of the skeleton key by Penny Warner

Sly the sleuth and the code mysteries by Donna Jo Napoli and Robert Furrow ; illustrated by Heather Maione