A Roman consulting the Sibylline Books was not asking what would happen. He was asking what to do. That distinction is the whole difference between Roman prophecy and the popular idea of it, and everything about the Cumaean Sibyl follows from it.
When a prodigy occurred — a rain of stones, a hermaphrodite birth, a statue that sweated, a wolf in the Forum — the Senate could order the priestly college of the quindecimviri sacris faciundis to open the Books. What came back was a prescription: institute this cult, hold this expiatory rite, send for this god from abroad. It was the Sibylline Books that brought Venus from Eryx, that founded Flora's games, that ordered the Great Mother's black stone imported from Asia Minor in 204 BCE, and that on at least two occasions in a crisis prescribed the live burial of a Gaulish and a Greek couple in the Forum Boarium.
They were, in effect, a technical manual for repairing the relationship between Rome and the gods. Nobody consulted them about the future.
Nine books, three burnings, one price
The acquisition story is told by Aulus Gellius, Dionysius of Halicarnassus and others, with minor variations. An old woman came to one of the Tarquin kings offering nine books at an enormous price. He refused; she burned three and offered the remaining six at the same price. He refused again; she burned three more and offered the last three, still at the price of nine. He bought them, and she vanished.
The surviving three were kept in a stone chest in the vault beneath the Capitoline temple of Jupiter, under armed guard, consulted only by decree of the Senate, and the college that guarded them grew from two members to ten to fifteen. Unauthorised consultation was a capital matter.
Cumae, and a tunnel that may not be hers
Cumae, on the Bay of Naples, was among the oldest Greek settlements in Italy, founded in the eighth century BCE, and the seat of a prophetess who had been famous for centuries before Rome mattered.
In 1932 Amedeo Maiuri excavated a long trapezoidal gallery cut into the tufa of the acropolis, over a hundred metres of it, with side openings admitting bars of light, ending in a rectangular chamber. He identified it as the cave of the Sibyl, and it fits Virgil's description well enough to have become one of the most visited sites in Campania.
The identification is now widely doubted. The gallery's construction resembles military works of the Greek and Samnite periods, and several specialists read it as a defensive or communication passage. The honest position: an ancient oracle at Cumae is well attested in literature, and whether this particular tunnel is its shrine is unresolved.
Virgil's version, and the leaves
The sixth book of the Aeneid gives the Sibyl her definitive portrait: Deiphobe, priestess of Apollo and Hecate, in a cave with a hundred mouths, seized by the god so violently that her voice and her features change, and, once Aeneas has plucked the golden bough, his guide down to the dead.
Earlier in the poem Virgil supplies the detail that everyone remembers: she writes her answers on leaves, arranged in order at the mouth of the cave, and if the wind gets in they scatter and she will not put them back. Aeneas is warned to beg her to speak aloud instead. A prophecy that is true, complete and irrecoverably disordered is a very good image for what Romans thought the sources of divine knowledge were actually like.
The jar
Ovid gives her the reason for her condition: Apollo offered her anything she wanted, and she asked for as many years of life as there were grains in a handful of dust — without asking for youth. Seven hundred years down, three hundred to go, and at the end she expects to be nothing but a voice.
Petronius takes it further, and grimmer. At Trimalchio's dinner party a guest claims to have seen the Sibyl at Cumae hanging in a jar, and when the local boys asked her what she wanted, she answered: I want to die. That line, quoted in the original Greek and Latin, is the epigraph to T. S. Eliot's The Waste Land — which is how a joke told by a vulgar freedman at a fictional Roman dinner became the doorway to the most influential poem of the twentieth century.
Fire, forgery and an audit
In 83 BCE the Capitoline temple burned and the Books burned with it. Rome responded by sending envoys around the Mediterranean — to Erythrae, to Ilium, to Africa, Sicily and the Italian towns — to collect Sibylline oracles and assemble a replacement. What Rome consulted for the next four and a half centuries was therefore an anthology, gathered by committee, and everyone knew it.
Augustus then audited the collection. He is reported to have burned some two thousand prophetic books circulating in Rome as spurious, had the remainder examined, and in 12 BCE moved the official corpus from the Capitol to a gilded chest beneath the statue of Apollo in his new temple on the Palatine. Prophecy had become politically dangerous during the civil wars, and the emperor took control of it as he took control of everything else. The collection is said to have been burned for good by the general Stilicho early in the fifth century CE — reported by a hostile contemporary poet, and a fitting end for a text nobody outside fifteen men had ever read.
What survives is not what Rome had
The Sibylline Oracles that exist today, in twelve or fourteen books of Greek hexameters, are a different thing entirely: a Jewish and Christian compilation from roughly the second century BCE onwards, using the Sibyl's voice for religious argument. They are genuinely interesting and they are not the Roman state's books, which nobody has read since antiquity.
The Sibyl's Christian afterlife was enormous. Because Virgil's fourth Eclogue, prophesying a child who would bring back the golden age, was read as a pagan prediction of Christ, the Sibyls were admitted as witnesses to revelation: the Dies Irae names David and the Sibyl in the same line, and Michelangelo painted five Sibyls on the Sistine ceiling alongside the Hebrew prophets, the Cumaean among them as a massive, muscular old woman with a book. A prophetess who told Romans which expiatory rite to perform ended up on the ceiling of the pope's chapel.
Questions the Books raise
Were the Books written in Latin?
No — in Greek hexameters, which is one reason the college that consulted them needed specialist interpreters and why Greek-rite cults were so often what the Books prescribed. The acrostic form is mentioned by Cicero, who notes that the verses were arranged so that the first letters of successive lines spelt out a message, and observes drily that this indicates careful composition rather than prophetic frenzy.
How many Sibyls were there?
Varro listed ten — Persian, Libyan, Delphic, Cimmerian, Erythraean, Samian, Cumaean, Hellespontine, Phrygian and Tiburtine — and his list became canonical, which is why Renaissance art depicts them in sets. Earlier Greek writers speak of a single Sibyl, and the multiplication looks like an attempt to reconcile competing local claims to the same famous figure.
Did ordinary Romans consult her?
Not the state Books, which were sealed and controlled. But freelance prophecy was everywhere — street oracle-mongers, collections of verses in private hands, and the lot-oracles like the one at Praeneste. The Roman state's repeated purges of unofficial prophetic books are the best evidence for how much of it there was.
Is the Tiburtine Sibyl connected to her?
She is a separate figure of the same type, located at Tibur, and she had a spectacular medieval career of her own as the prophetess who supposedly showed Augustus a vision of the Virgin and Child and told him a greater king had been born. That legend is medieval, not Roman, and it explains why the Ara Coeli church stands on the Capitoline — but it is a good illustration of how completely the Sibyl was converted along with everyone else.