Book 6 "IN TOO DEEP" has numbers on the plane in the front cover the code for that is "find the other Ekaterina who came too close" Hop that helps
There are no cheat code in any game in the 39 clues.
The eighth book in The 39 clues series is 'The Emperor's Code.'
The title of the seventh 39 clues book is The Viper's Nest. My agent Code is P2J34GKXXP on the 39 clues website.
P3rw4fpnxp
kf3x4ttct3
246rp6fxxp
water
246RP6FXXF
P29rp379xp
LUKE1
its PAGU32CVF3
G697hgv4f2