My wife is reading Rubicon which retells the "myth" of the Cumaean Sibyl. Is there any evidence that her prophecies existed, and is there any evidence of what was said? Rubicon quotes from the prophecies - from where did the author obtain that text?
First of all, don't bother with Rubicon. It's not history, and it's written neither by a historian nor a classicist, or even by someone with a good enough grasp of Latin not to rely entirely on translations. It's really about the political climate immediately following 9/11, thinly disguised as a history of Caesar's lifetime and the end of the Republic, and it's full of nonsense and constant errors. It's many failings have been discussed by other classicists to death on here, so I'll leave it with that.
Why should we doubt that the Sibylline books were real? They're mentioned by writers until very late and they're all very consistent with each other. Mind you, the Sybilline books and the Sibylline Oracles are two totally different things--the former is a collection of hexameter verses dating g from early antiquity which, except quoted fragments, have all been lost; the latter is a collection, copying the form of the former, of much later verses supposedly collected from the living Sibyls of late antiquity and are heavily influenced by Christian thought (often called the Pseudo-Sybilline Oracles to help further distinguish them).
The existence of the Sybilline books is pretty well attested, although since they were a well-guarded part of a particular magisterisl office nobody could read them. After all, it was on the advice of the Sibyl that a number of rites and traditions were begun, most notably the introduction of the Cult of Cybele from Phrygia. The Sibylline books were not some sort of Delphic Oracle, as many people (including Holland, as I remember) think, that advised in vague terms on how to avert disaster and save the city in times of grave danger. The books set out, rather straightforwardly, the religious rites to perform should certain omens appear. Sibylline verses are quoted by a number of authors, often in a great deal of completion. Livy and Suetonius both quote or, more often, paraphrase the books, as do Cassius Dionysus, Plutarch, and a number of others. This I presume is what Holland is quoting, or a transition of them, as of course the physical books were lost
She's mentioned in The Aeneid by Virgil, in Ovid's Metamorphoses, and other texts.
I'd start with the [Sibylline Books] (http://en.wikipedia.org/wiki/Sibylline_Books).
Also the [Cave of the Sybil] (http://www.atlasobscura.com/places/cave-of-the-sibyl-antro-della-sibilla) is interesting as well.