We know of two "Sibylline" works: the Sibyllini libri and the Sibyllini oracula. The former was the oracular text supposedly entrusted in the Tarquins and consulted during prodigies and crises. Apart from a handful of quotations from two fragmentary authors nothing of it survives. The latter is essentially worthless for our purposes, it's a pseudoepigraphical late antique Christian text that mostly contains prophecies of the destruction of various Roman cities, Christian doctrinal maxims, Jewish philosophical discussions, prophecies of apocalypse, Jewish oracles--you get the idea.

We know next to nothing about the way the Sibyl's oracles were written. Dionysius and Cicero both attest, however, that the genuine Sibylline oracles were usually acrostic. According to Cicero the initial letters of the hexameter verses spelled out the first line of the prophecy--Dionysius confirms the acrostic nature of the hexameters, noting that when the books were destroyed in a fire they were reconstituted from copies from throughout Italy, with a major criterion to determine genuine oracles and false ones being the acrostic nature. Interestingly, there's only one oracle in the Christian Sibyllini oracula that employs acrostics, a poem that predicts the birth and crucifixion of Christ--generally the Christian text doesn't even really pretend to be actually from the Sibyl, so this is maybe a little noteworthy. The supposed Sibylline quotations preserved in Phlegon doesn't tell us much. Phlegon seems to support the idea of acrostic oracles (although his second oracle is not acrostic, and Virgil's Fourth Eclogue, which some people think is an imitation of the Sibyl's style, contains no acrostic verses except one that--presumably accidentally--says cacata, "having been crapped"), and the Greek is pretty weird--there's a translation by Hansen, but there's all kinds of weird and ambiguous features in the Greek that simply don't come out in the English. The more complete of Phlegon's prophesies, the first (I believe the second, which is more fragmentary, is thought by some people to actually be two separate prophecies that were combined), was apparently delivered in 125, B.C. It predicts the birth of a hermaphrodite and prescribes a number of sacrifices, as well as their exact formula. Both of these prophecies are quite unambiguous, except in the syntax of the Greek, which like I said gets pretty weird. This is, however, pretty typical of the reports of Sibylline oracles that we receive. Generally we're told that the Sibylline books contained verses that predicted various prodigies: volcanoes, the sieges of cities, various extraordinary natural phenomena. They then gave instructions on rites to follow to deal with these prodigies. The prodigies appear to have been relatively general--they're not really "prophecies" in that they don't say "on such-and-such a day this is going to happen here and this is what you're going to do about it." They're also usually not about important (at least politically important, though they're of course ritually important) things, they're about stuff like hermaphrodites being born or rivers running with blood. Compare this to two supposed oracles of Marcius, a Latin seer whose prophecies were also cataloged, that Livy claims to paraphrase. Read in 216 they (according to Livy, who presents a very cleaned-up version) very clearly and specifically predict the disaster at Cannae (or at least a military disaster at the same place) and the expulsion of an invading enemy. This is very unlike the reports of the Sibylline books that we get from our authors, who are pretty much unanimous in their transmission of Sibylline oracles dealing with natural and supernatural prodigies, not of political or military events. In this respect the Sibylline books appear to have been quite specific--they appear to have detailed exactly what the prodigy would be and what precisely to do about it--but they're not specific in the kind of way that I think you mean, in that they really don't seem to give a shit about the events that they were connected with. Prodigies were expected to happen during and before times of crisis, as expressions of the gods' will. In 125 Gaius Gracchus was on the eve of beginning his major political career (his tribunate was in 123), and in that year Fulvius Flaccus tried to force the urban population to accept his rural land distribution project, without success. The connection between the prophecy (and appearance) of a hermaphrodite and the tension in the state would have been obvious to a Roman, even though the Sibyl did not explicitly mention the Gracchi or political tension