In Lord of the Rings, Frodo leaves Sam the Red Book of Westmarch, in which to record the goings on of Middle Earth after he is gone. Tolkein himself says that his inspiration for the fictional book was the Red Book of Hergest in the Bodleian Library at Oxford, which he knew well.
In Wales, there were three such books of which we know:
The Red Book of Hergest
The Black Book of Camarthan
The White Book of Rhydderch
The Red Book of Hergest was written between 1375 and 1425 by Hywel Fychan fab Hywel Goch of Fuellt, for his employer, Hopcyn ap Tomas ap Einion of Ynys Tawe. In it are some of the most famous Welsh texts, including the Chronicles of the Princes, The History of the Kings of Britain by Geoffrey of Monmouth, The Ruin and Conquest of Britain, the Four Branches of the Mabinogi, and so on. The complete list is here: http://www.maryjones.us/ctexts/hindex.html
The Black Book of Camarthen, in the National Library of Wales (Peniarth Manuscript 1), dates to the mid-thirteenth century and is believed to have been the work of a single scribe at the Priory of St. John in Carmarthen. It is one of the first works written wholly in Welsh and comprised mostly of poetry, primarily on the subject of Dark Age (sorry, Brynne) topics. The contents of which are here: http://www.maryjones.us/ctexts/bbcindex.html
The White Book of Rhydderch contains much of what is in the other two books, with an emphasis on religious subjects and prose, rather than poetry. The copy in the National Library of Wales dates to around 1350 AD. It is found here: http://www.maryjones.us/ctexts/windex.html
As to how old the material in the books actually, it is not clear, or from what earlier books they were copies. Scholars date the version of the Four Branches of the Mabinogi that is in these books to around 1100 AD, given the linguistic characteristics, but that is not to say that the stories aren’t older. Much of the poetry is much older–dating to between 400-700 AD for the Dark Age poets such as Taliesin and Aneurin.