The Code Busters Club: Books 2-6 by Penny Warner (Ex-Library HC/DJ)

Cody, Quinn, Luke, and M.E. may not have much in common with each other, but they do love playing around with codes. In fact, they love codes so much, they have their own private club, with a super-secret hideout and passwords that change every single day. This exciting interactive mystery series offers more than fifteen codes for you to decipher, including the Consonant code, Morse code, and American Sign Language. Test your brain with the Code Busters and solve the mystery along with them. Answers are in the back, if you ever get stuck.