* Focus on Adventure: The primary focus of the series is on the adventurous journey of the Cahills, solving puzzles and uncovering secrets.
* Fictionalized History: The series uses real historical events and figures, but often weaves them into a fictional narrative, sometimes embellishing or re-imagining them.
* Time Travel Element: Some books in the series involve time travel, which further blurs the line between historical accuracy and fictional storytelling.
Therefore, the 39 Clues series can be categorized as adventure fiction with historical elements. While it uses historical events and figures as a backdrop, it's more about the fictional adventure and the mysteries surrounding the Cahill family.