In the back of many The 39 Clues books, there are advertisements for other book series. These book series include Infinity Ring, Spirit Animals, Tomb Quest, and Shadow House. In Worlds Collide, a fan-written and Admin edited additional book, The 39 Clues characters are mashed up with Infinity Ring and Spirit Animal characters to stop villains from all three series.
Shatterproof[]
This is the first book that an advertisement appears in. The advertisement is for Infinity Ring.
Trust No One[]
Trust No One also advertises Infinity Ring.
Day of Doom[]
Day of Doom also advertises Infinity Ring.
Nowhere To Run[]
Nowhere To Run advertises Spirit Animals.
Breakaway[]
Breakaway advertises Spirit Animals.
Countdown[]
Countdown advertises Infinity Ring.
Flashpoint[]
Flashpoint advertises Spirit Animals.
Mission Titanic[]
Mission Titanic advertises Spirit Animals.
Mission Hindenburg[]
Mission Hindenburg advertises Tomb Quest.
Mission Hurricane[]
Mission Hindenburg advertises Tomb Quest.
Mission Atomic[]
Mission Atomic advertises Tomb Quest.
Outbreak[]
Outbreak advertises Shadow House.