The 39 Clues: Doublecross Series is the 4th story arc in the 39 Clues Franchise, sequel to Unstoppable Series. It features 4 exciting adventure books for young adult readers, published between 2015 and 2016. The books are written by four different authors and among them, only Jude Watson has previously written books in the series. For the other three authors (C. Alexander London, Jenny Goebel, and Sarwat Chadda), their books are their first in the 39 Clues series.
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