Collection of prophecies revered by Ancient Rome, and consulted by the monarchy, republic, and empire in times of natural disaster for instructions on how to appease the gods.

The story of how the books came to Rome is a famous one:

In ancient annals we find this tradition about the Sibylline Books. An old woman, a perfect stranger, came to king Tarquin the Proud, bringing nine books; she declared that they were oracles of the gods and that she wished to sell them. Tarquin inquired the price; the woman demanded an immense and exorbitant sum: the king laughed her to scorn, believing her to be in her dotage. Then she placed a lighted brazier before him, burned three of the books to ashes, and asked whether he would buy the remaining six at the same price. But at this Tarquin laughed all the more and said that there was now no doubt that the old woman was crazy. Upon that the old woman burned up three more books and again calmly made the same request, that he would buy the remaining three at the original figure. Tarquin now became serious and more thoughtful, and realising that such persistence and confidence were not to be treated lightly, he bought the three books that were left at as high a price as he had been asked for all nine. Now it is a fact that after then leaving Tarquin, that woman was never seen again anywhere. The three books were deposited in a shrine and called "Sibylline"; to them the Fifteen resort whenever the immortal gods are to be consulted as to the welfare of the State.
--Aulus Gellius, Noctes Atticae. I, xix. (Translated by John C. Rolfe, Loeb Classical Library, 1927.)
(Douglas Adams tells the story beautifully as an epilogue to Last Chance to See. The Douglas Adams rendition is reprinted widely on the Web, often without attribution or erroneously attributed to Daniel Quinn.)

Though sold to the Romans by the mysterious woman, who turned out to be the Cumaean Sibyl, the books were understood to be the transcriptions of the original Erythraean sibyl. Written in Greek hexameter, the original books were interpreted by a revered body of priests/librarians (first 2, later 15). For 500 years, Rome consulted them when signs of divine unhappiness appeared: an earthquake in 461 BC, an outbreak of distemper in 399 BC, a shower of stones falling from the sky (343 BC), and other miscellaneous lightning strikes, speaking animals, eclipses, and springs running with blood. Although the priesthood guarded the books and studied the verses, the books were only consulted by order of the Senate. The actual consultation and interpretation may have varied over the years. One method involved the librarians in charge choosing a line at random, and an acrostic was made from the words in this line. Then verses beginning with those letters were discovered in the oracles and put together for instructions on how to please the gods.

When the books, stored under the temple of Jupiter Capitolinus, were destroyed by fire in 83 BC, Augustus authorized a commission to go out and gather another collection of oracular utterances. Approximately 1000 lines of the original Erythraean pronouncements were found, and Augustus and Tiberius's commissions worked on recreating the sibyl's canon, weeding out Jewish and pagan (Egyptian) propaganda (Christianity would also later jump on the sibylline bandwagon and claim that their religion was prophesized by the sibyls, as proof of its truth). These new books were stored under the temple of Apollo on the Palatine.

While there are 12 extant "books" in Western literature, they include Jewish and Christian revisions. Numbered by publishers I-XIII (based on a manuscript in Augsburg and published in 1585) and XI—XIV (added in the 19th century from Vatican manuscripts), they are mostly studied now for insight into competing theologies during the Roman era. The prophecies themselves are vaguer than those of Nostradamus, with specific historical incidents and persons that do appear in the books evidence of tampering by Christian and Jewish writers.

The early Christian Church venerated these books (even up until St. Augustine), because of an acrostic that appears in them, ΙΧΘΥΣ, which seems to predict the coming of Jesus Christ. (Book VIII).



Sources:
"Ancient Oracles: The Cumaean Sibyl," Morgana’s Observatory, "<http://www.dreamscape.com/morgana/desdemo2.htm> (27 March 2002)
Aulius Gellus, Attic Nights. I, xix. Translated by John C. Rolfe, Loeb Classical Library, 1927. <http://members.aol.com/zoticus/bathlib/attic/book1.htm> (18 April 2002)
Bate. H.N. "Introduction to The Sibylline Oracles, Books III-V." New York: Macmillan, 1918. At Bob Kobres Personal Site. <http://abob.libs.uga.edu/bobk/sib/sib.html> (5 July 2002)
Gill, N.S. "Cumean Sibyl." Ancient/Classical History Glossary– About.com, < http://ancienthistory.about.com/library/bl/bl_cumaean_sibyl.htm> (27 March 2002)
Hare, J.B. "Introduction to The Sibylline Oracles." Internet Sacred Text Archives. <http://www.sacred-texts.com/cla/sib/> (18 April 2002)
Healy, Patrick J. "Sibylline Oracles." Catholic Encyclopedia, Vol. XIII, Robert Appleton Company, 1912. <http://www.newadvent.org/cathen/13770a.htm> (27 March 2002)
Lactantius, Lucius Caecilius Firmianus. "Of Divine Testimonies, and of the Sibyls and their Predictions," Divinæ Institutiones. I, vi. Found at The Fathers of the Church. <http://www.newadvent.org/fathers/07011.htm> (18 April 2002)
Lemprière, John. A Classical Dictionary. London: T. Cadell, 1788. Facs., Garland, 1984. Quoted in Steven E. Jones, "Sibyllae," Mary Shelley, _The Last Man_ - Electronic Editions, Romantic Circles, 2001, < http://www.rc.umd.edu/editions/mws/lastman/sibyl.htm> (27 March 2002)
"Anonymous Preface to the Sibylline Oracles." Quoted in Milton S. Terry, The Sibylline Oracles,1899. Internet Sacred Text Archives. <http://www.sacred-texts.com/cla/sib/sib15.htm> (18 April 2002)