The first book in the 39 Clues series is called, The Maze of Bones, by Rick Riordan. He does a great job setting up the premise of the story. Two orphans must choose whether they want to take an inheritance or accept a quest. They accept the quest. There is peril and intrigue as soon as they begin the quest. Mysterious characters are introduced and little known history is woven seamlessly into the story. I found it very interesting.
Each chapter focuses on the perspective of a different character. This approach to the story occasionally interrupted the flow for me.
This is an excellent book series for any adventurous child that enjoys reading a good mystery.