I don't know the answer on the inside page, but several pages have Morse cod eon them. Decoded it says "There is another madrigal watcher"
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
ARCTIC
kf3x4ttct3
246rp6fxxp
water
246RP6FXXF
P29rp379xp
The Viper's Nest
its PAGU32CVF3