By Rick Riordan
A brother and sister are handed a choice at their grandmother's funeral: a million dollars, or the first clue in a worldwide treasure hunt their whole ruthless family is racing to win.
At the funeral of their beloved grandmother Grace, Amy and Dan Cahill learn that their family is the most powerful in history, and that Grace has left every relative the same choice: take a million dollars and walk away, or take a single clue and join the hunt for the thirty-nine clues that lead to the source of the family's power. Amy and Dan burn the money and take the clue. Now they're chasing a trail that runs from Philadelphia to the catacombs beneath Paris, one step ahead of cousins, aunts, and uncles who will cheat, sabotage, and worse to win first, with only their au pair and their wits to keep them alive.
Rick Riordan built this to move, and it does. The chapters are short, most of them end on a fresh problem or a slammed door, and the puzzle keeps unlocking just enough to make a child feel smart. Amy and Dan are a believable brother and sister, bickering one minute and covering for each other the next, and they're the smallest, poorest players in a hunt full of grown-ups with money and menace, so you can't help rooting for them. Real places and real history get woven into the clues, so a child picks up Benjamin Franklin and the Paris catacombs without noticing they're learning. There are ten more books, each by a different author, so a hooked reader has a long road ahead.