Welsh: The Book of Vergil
A non-existant text of druidic magic. It is first mentioned in the Hanes Taliesin, wherein Cerridwen is reading from the book in order to find the potion which will give Afagddu all sorts of inspiration. This particular text only dates to the fifteenth or sixteenth centuries.
It is later referenced (IIRC) by Iolo Morganwg, that infamous Welsh forger and Romantic neo-Druid in his Barddas, a very unreliable text of "druidic theology."
Finally, it is best known recently thanks to Douglas Monroe's insistance in his Merlyn books that it is a secret book of druidic magic.
As defined, the name means "Book of Vergil." It is common knowledge that Vergil was attributed magic and other such powers during the Middle Ages (just read Dante), and so it is no surprise that a magic book would be attributed to him. However, Vergil was not a druid (though he may have grown up around some Alpine Celts).
Secondly, the druids did not, as a rule, write down their beliefs or practices. This is as much as established fact. The idea that some druid book of magic even exists is laughable, much less one written by Vergil!