The Lightning Thief
by Rick Riordan
The Oracle can see into the future and can offer prophecies, or predictions. She lives in the attic in a decaying mummy's mouth and her prophecies take the form of a magical green mist. While she does give valuable insight into the future, she also kind of speaks in riddles, making it hard to really know what she means. Percy is baffled when she tells him he will fail to save what matters most and that he will be betrayed by someone who calls him a friend.