A little boy moves into or near a huge old mansion with a tall circular staircase. A girl (possibly named Cecilia/Celia) had died there after supposedly accidentally falling down the stairs. An old caretaker woman who gave her medication becomes a suspect, but later they discover the girl had secretly hidden her pills in a grandfather clock wrapped in a napkin/paper with initials on it. She intentionally created a scavenger hunt with clues involving mirrors/light, dusty rooms, floor tile symbols, and diamond earrings hidden in the chandelier. The boy sometimes imagines seeing her among children playing. At the end they realize she actually committed suicide by jumping from the staircase and the clues were meant to reveal the truth and clear the caretaker
(not sure all from memory)( I looked everywhere for it)