The Black Book of Carmarthen (Llyfr Du Caerfyrddin) & The Red Book of Hergest are available online.
Currently housed at the National Library in Wales, the Black Book of Carmarthen (Peniarth MS 1) is a manuscript dating to the middle of the thirteenth century. It is believed to have been the work of a single scribe at the Priory of St. John in Carmarthen.