A couple years ago, I started reading The 39 Clues series. It was enjoyable, both with strengths and weaknesses. I stuck with the series as each book came out, and I haven't been disappointed. The books are fun, and the initial awkwardness of the first three books is gone. The authors eventually figured out how to settle into unified roles of Amy and Dan Cahill, and even Nellie (their au pair). They authors even figured out how to make the characters grow in interesting ways, keeping them feeling consistent. I have to say that I'm impressed. Based on the first three books, I thought the series was going to be disjointed all the way through. Not so at all.
The adventures are fun and entertaining, and I often read each book in a day. And then was looking forward to when the next one would come out. I was a little worried about the fact that there were 39 clues but only ten books, however I really like the way the series ended. The concepts and themes introduced are ones I find interesting anyway, and I like the way the series explores them.
My eight year old son discovered these books earlier this year. He and a few of his classmates love them, and he's read them all. He's even moved onto the next series, Cahills vs. Vespers. I read the 39 Clues book 11, which is basically setup for Cahills vs. Vespers, and am intrigued. I think I will be reading this next series with my son. :)