Annotated Welsh in Three Months by Phylip Brake and Mair ap Myrddin (DK Publishing)

However, if Welsh does not yet possess a spoken standard, it does possess a literary standard which can be traced back to the translation of the Bible by Bishop WIlliam Morgan in 1588, which in turn is based on the language of the medieval court poets who were the heirs of the Cynfeirdd, the earl poets Aneirin and Taliesin. These lived in the sixth century AD and described battles which took place in today's Scotland and Northern England [...]

Annotated The Mabinogion (Oxford University Press)
Sioned Davies is Chair of Welsh at Cardiff University. Her special interest is the interplay between orality and literacy, together with the performance aspects of medieval Welsh narrative. 
Oh! This is fascinating. Perhaps some interesting tidbits for my growing theory about the borders of orality and literacy could be hiding in some of her research?
Bookmarked The Mabinogion Translated by Sioned Davies (global.oup.com)
Celtic mythology, Arthurian romance, and an intriguing interpretation of British history--these are just some of the themes embraced by the anonymous authors of the eleven tales that make up the Welsh medieval masterpiece known as the Mabinogion. They tell of Gwydion the shape-shifter, who can create a woman out of flowers; of Math the magician whose feet must lie in the lap of a virgin; of hanging a pregnant mouse and hunting a magical boar.
I’ve looked at versions of this before, but this looks like one of the more interesting translations to read. 

(referred by Top 10 goddesses in fiction

Liked a tweet by @k_j_turner (Twitter)
I love the bridge that book formats like this can provide in learning new languages. Reminds me a bit of some of the Folger Shakespeare annotated plays that helped to define words, terms, and culture that have changed significantly since they were written. 

How does one find more of these?

Read - Want to Read: Dysgu Cymraeg ( Y Ganolfan Dysgu Cymraeg Genedlaethol)
Hwn yw'r cwrs cenedlaethol ar gyfer oedolion sy'n dysgu'r Gymraeg ar lefel Mynediad (A1). Mae'r llyfr i'w ddefnyddio mewn dosbarth. Mae pob uned yn cyflwyno thema wahanol, ynghyd a geirfa a phatrymau iaith newydd. Mae'r cwrs yn datblygu'r pedair sgil iaith - siarad, darllen, ysgrifennu a gwrando. Mae adran Gwaith Cartref ym mhob uned. This is the national Welsh for Adults course book for learners at Entry level (A1). The book is intended for use in a classroom. Each unit looks at a different theme and introduces new vocabulary and language patterns. The course will provide practice for developing all four language skills - speaking, reading, writing and listening. There is a homework section in each unit.