The Throne of Fire by Rick Riordan

That Rick Riordan is a busy writer! After you've read the 39 Clues, and of course all the Percy Jackson books, start the Kane Chronicles. Carter Kane and his sister must prevent the chaos snake Apophis from breaking free in a few days' time or the world will come to an end. To have any chance of battling these Forces of Chaos, the Kanes must revive the sun god Ra. Book 2 in the Kane Chronicles series. Summary from the MARINet catalog.

39 Clues: The Maze of Bones by Rick Riordian

What would happen if you discovered that your family was one of the most powerful in human history? What if you were told that the source of the family's power was hidden around the world, in the form of 39 clues? What if you were given a choice - take a million dollars and walk away ... or get the first clue? If you're Amy and Dan Cahill, you take the clue - and begin a very dangerous race. Check it out from the library: The Maze of Bones.
Summary from the MARINet catalog.