The Book of Aneurin
"13th century Welsh manuscript, now held by the Cardiff Library (Cardiff MS 1)." --our own aneurin.
Anuerin was a contemporary of Taliesin and Myrddin, and thus one of the Hengerdd. 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 The 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), Owein ap Urien (Chretien's Yvain), and Taliesin are all named in the poem. In fact, there is a reference to King 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 narrative poems). Unfortunately, I have yet to find a full translation, as there have been none. (The Gwarchan Maelderew, supposedly written by Taliesin and mentioned in his poem "Cad Goddeu", is the same as that which Douglas Monroe spuriously calls a book of Druidic magic, the Gorchan of Maeldrew--it is a lament on a fallen warrior, not a book of spells.)
Here are the poems: