The Book of Aneurin
Cardiff MS 2.81
Aneurin was a contemporary of Taliesin and Myrddin (Nennius mentions him under the name "Neirin" from the alternate spelling "Aneirin"). A poet in Urien's court, he was present at the battle of Cattraith, ca. 600 CE/AD in Catterick, Yorkshire, on which he wrote his epic poem Y Gododdin, about the defeat of the Britons against the Saxons. He is critical in some places of the rash behavior of the soldiers; in other places, he mentions knights familiar to us from Arthur's court: Peredur (from the Mabinogion, the Welsh name of Perceval), Owain (Chretien's Yvain), and Taliesin are all named in the poem. In fact, there is a reference to Arthur as a mighty warrior of the recent past.
Anuerin also is supposed to have written a number of gwarchanau (Welsh: lais, that is, short stories). Unfortunately, I have yet to find a full translation, as there have been none. (The Gwarchan Maelderew, supposedly written by Taliesin, which Douglas Monroe spuriously calls a book of Druidic magic, the Gorchan of Maeldrew--is a lament on a fallen warrior, not a book of spells.)
The manuscript we have here--Carddif MS 2.81--is usually dated to around 1255-1265. The poems it contains are thought to have been transmitted orally long before ever being transcribed, perhaps in the 9th or 10th century; this may account for the variant readings of some stanzas, which are found after the gwarchanau section.
Digital images of the manuscript can be found at Gathering the Jewels, a digital collection of the Cardiff Library, where the manuscript now resides.
Here is the manuscript, as I have reconstructed it through referencing the edition of The Four Ancient Books of Wales and the digital edtion mentioned above: