Aulus Gellius, Attic Nights 1.19
19. The account of the Sibylline Books and King Tarquin the Proud
This story is preserved in the ancient accounts concerning the Sibylline books. An old woman, unknown, approached king Tarquin the Proud with new books which she was claiming were divine oracles (and she wished to see them). Tarquin asked the price. The woman asked for an enormous, excessive amount. The King, as if he believed she was senile, laughed. Then she placed a brazier already lit before him, burned three of the nine books and asked whether the King wished to buy the remaining six for the same amount. But Tarquin laughed even more and said that he’d lost all doubt that the woman was insane. She woman then burned up three more books immediately and calmly aked him the same thing again, to buy the three remaining books for that price. Tarquin then became more serious and attentive, believing that this insistence and confidence ought not to be ignored: he bought the remaining books for no less than the price which had been sought for all of them!
But it is agreed that after the woman departed from Tarquin, she was never seen again. The Three books, which were placed in a shrine, are called “The Sibylline Books”. The Fifteen [priests] turn to them for oracles whenever the gods must be consulted for the public good.”
XIX. Historia super libris Sibyllinis ac de Tarquinio Superbo rege.
1 In antiquis annalibus memoria super libris Sibyllinis haec prodita est: 2 Anus hospita atque incognita ad Tarquinium Superbum regem adiit novem libros ferens, quos esse dicebat divina oracula; eos velle venundare. 3 Tarquinius pretium percontatus est. Mulier nimium atque inmensum poposcit; 4 rex, quasi anus aetate desiperet, derisit. 5 Tum illa foculum coram cum igni apponit, tris libros ex novem deurit et, ecquid reliquos sex eodem pretio emere vellet, regem interrogavit. 6 Sed enim Tarquinius id multo risit magis dixitque anum iam procul dubio delirare. 7 Mulier ibidem statim tris alios libros exussit atque id ipsum denuo placide rogat, ut tris reliquos eodem illo pretio emat. 8 Tarquinius ore iam serio atque attentiore animo fit, eam constantiam confidentiamque non insuper habendam intellegit, libros tris reliquos mercatur nihilo minore pretio, quam quod erat petitum pro omnibus. 9 Sed eam mulierem tunc a Tarquinio digressam postea nusquam loci visam constitit. 10 Libri tres in sacrarium conditi “Sibyllini” appellati; 11 ad eos quasi ad oraculum quindecimviri adeunt, cum di immortales publice consulendi sunt.
The Sibylline books had 15 priestly interpreters by the time of Cicero. Why? Maybe because they were in Greek!