Someone posted a link yesterday to an ebook on the resources page of the National Welsh College.
Among other things there, I noticed a book (from 1996) about Welsh dialect which looks quite interesting:
https://llyfrgell.porth.ac.uk/media/tafodiaith-yn-y-gymraeg-g-m-awbery-1996-3
(The download link is near the top-right of the page: )
It’s all in Welsh, but doesn’t look that difficult to follow what’s going on and there are supporting maps, word-lists and some video clips.