Read - Want to Read: Modern Welsh by Gareth King (Routledge)
This new expanded edition of Modern Welsh is the ideal reference source for all speakers and learners of Welsh, suitable for use in schools, colleges, universities and adult classes at all levels. Focusing on contemporary spoken Welsh, this new enlarged edition features a substantial new function-based section, explaining and exemplifying a wide range of sentence and phrase patterns. Notes on variations between dialects and between spoken and formal written forms have also been expanded. The Grammar presents the complexities of Welsh in a concise and readable form. Common grammatical patterns and parts of speech are discussed in detail, and extensive cross-references make the book comprehensive and easy to use.
I started this back in September, but hadn’t bookmarked it?
Listened to SaySomethingIn... Welsh: Level 1 Introduction from saysomethingin.com

This is pretty good and generally intense as a mental workout. Not too dissimilar to Duolingo, but it doesn’t have any visual portions for looking at or reading words. Perhaps in conjunction with Duolingo it would be a good speed course?

This particular post also makes me want to have a “study post” type/kind on my website. I’ve generally not been tracking it directly for things that aren’t otherwise reading, but it could include writing, listening, speaking, or otherwise working on educational related things that one might want to track: i.e. “how much time did I spend studying subject x?”

Bookmarked BBC Wales - Learn Welsh the Big Welsh Challenge (bbc.co.uk)
Suitable for complete beginners, this will support the learning you do in class or elsewhere. There are different topics ranging from greetings to discussing leisure interests. You can choose video subtitles or speech bubbles in Welsh or English to support your learning. A tutor guide will help you though the language clips, giving you an opportunity to practise. Learn Welsh wherever you go with MP3 audio and paper printouts. In time this will be a comprehensive course allowing you to progress in your learning.
Sadly, this website is painfully out of date and requires Flash.
Read Language in England and Wales - Office for National Statistics (ons.gov.uk)
Using data from the 2011 Census, we take a closer look at language within England and Wales. Those who reported English (or Welsh in Wales) as their main language accounted for 92.3% of the population, except in London where proportion was much lower. Those who reported another main language accounted for 7.7% of the population, with Polish topping the list of "other" main languages. London and the West Midlands saw the highest percentage of people who could not speak English "well" or "at all".
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