The Sibylline Books are collections of prophecies that were used during Ancient Rome and of which only fragments have survived, the rest being lost, deliberately destroyed, or misplaced. Ella is the only known reader of the books and Octavian was desperately after them.
Passed down from an oracle named the Sibyl of Cumae, the Sibylline Books are a collection of oracular utterances. The myth says the Sibyl came to Tarquin, the last Roman king, and offered nine books for an immense price. Tarquin simply sent her away and continued doing his work. The Sybil burned three of the books and offered them for the same price, which was promptly rejected. She burned three more and did the same thing with the same reaction. She burned two more and offered the last one for the same price. The king was very curious and he could not believe someone would offer him one book for such a huge price and finally decided to buy it. All of the prophecies affecting the Roman Empire were said to have been in these books, but they have been lost for centuries.
The Sibylline Books are first mentioned when Percy Jackson asks if Camp Jupiter had an Oracle so they could be given a prophecy. Octavian, the augur, remarks that the idea of an Oracle was cute because the camp has never had one. Hazel Levesque then explains to Percy that the Sibylline Books are books of prophecy and that ancient Rome would consult them whenever there were any major disasters. However, it is widely believed the books burned when Rome fell.
Octavian counters that only some people believe that the books burned, but Reyna Avila Ramírez-Arellano would not authorize a quest (because she is not stupid, according to Hazel). Camp Jupiter has a few scraps of the books safe, engraved on the marble floor of the Temple of Jupiter. One of these prophecies included the Prophecy of Seven, which Percy understands because Rachel Elizabeth Dare had recited it after becoming the Oracle.
Later, after Mars appears to the members of Camp Jupiter, he issues a quest. Octavian approaches him and says they need a prophecy from the Sibylline Books in order to issue a quest. Mars instead writes a prophecy for the quest and says they could add it to their books or engrave it on their floor with the other Sibylline Book's prophecies.
While on their quest to Alaska, Percy, Hazel, and Frank come across a Harpy named Ella, who is being tortured by the blind seer Phineas. While asking her questions, Ella speaks a few lines of prophecy, including the first Great Prophecy, and later, a prophecy about a son of Neptune drowning, but comments that the rest of the page was "burned." Hazel makes the connection that the burned book of prophecies Ella was talking about could be the Sibylline Books and concludes they need to keep her away from Octavian.
After returning to Camp Jupiter and defeating the Giant Army and Polybotes, Percy is elected Praetor, taking away Octavian's chance for power. Percy, Hazel, and Frank agree to protect Ella from Octavian, as with Percy now praetor, Octavian would look for other ways to extend his influence at Camp Jupiter, most likely by using his power of prophecy, and with Ella's knowledge of the Sibylline Books.
While the Greeks and Romans are talking about the Prophecy of Seven, Ella recites a page from the Sibylline.
In Jason's dream, Rachel Elizabeth Dare told Octavian and Reyna that Ella has been murmuring about the Sibylline Books and that she is the only source to it.
Ella, Tyson, and Rachel frequently visit Camp Jupiter to attempt to recreate the Sibylline Books.
It is mentioned that Ella is at Camp Jupiter helping recreate the Sibylline Books.
- It is hinted that Ella has memorized parts of the Sibylline Books as she also recites prophecies. It is unknown however where Ella read the book.
- While the Sibylline Books are filled with prophecies that predict important events during the Roman Empire, Ella remembered reading the first Great Prophecy, a prophecy relating to the Greek demigods of Camp Half-Blood.
- Octavian states that the books foretold the Prophecy of Seven for several thousand years while Greek demigods have only known about the prophecy for a little less than a year.
- The books are said to predict the fate of Rome, hence Octavian's strive to obtain them.
- The Sybilline books may possibly be named after the English word Sybil, which is a name for women who can predict the future (or vice-versa).