The 39 Clues Series
Comments
We really enjoyed the first 9 books in the 39 Clues series. After that, the 6-book Cahills vs. Vespers followed by the four-book series, the other four-book series, and the other loose books... let me just recommend that you don't approach this series with a competionist-mindset.
This sounds similar to Magic Tree House but for older kids. My son would love this!

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