Not really a review, but I was asked for a opinion, so:
I'm nowhere near a qualified scholar of anything Welsh; therefore, I probably missed a lot of intricacies. On the top of that, I read the translation that is generally viewed as inferior. But needs (student's ever-empty pockets) must, so free ebook on smartphone during the commute it was.
Things I noticed:
-Invisibility and magic cauldrons play a great part, even in latter, Christian-influenced works. There are some elaborate illusions as well.
-The order is baffling: we start with Arthuriana (three romances plus two of "native tales"), then jump to paganic Four Branches (and shift in the style is very apparent), then turn back to "native tales". Why wasn't the order chronological? Or if you wanted to put more appealing stories in the front, why not shift mythology to the end? It's not just nitpick, shift in style twice over feels quite awkward.
-First, there were sudden lists of The Silmarillion. Then I read some actual sagas (great source of inspiration) and saw that I got off lightly back then. And then I read Culhwch and Olwen and saw how naive I was.
-Lady Guest mentions possible influences of early medieval Welsh literature on latter medieval romances in the introduction, but I was more stunned by the influence on Fairy Tales. Above-mentioned Culhwch and Olwen alone contains a lot of motives, though there is more to be found in others. I even noticed potential influences to a few folktales from my own region; northernmost parts were inhibited by Celts during the period of their greatest expansion.
-Is it bestiality if they are both shape-shifted and possibly under compulsion to act like animals? Presuming compulsion, is it incest? Presuming one of two is temporarily shape-shifted into a female animal, is it homosexual? (Since it's likely they are aware of what's happening to them - it wouldn't be a punishment otherwise.)
-Will no one think of the children? (No, seriously. A lot of people decry treatment of female characters in olden works, but here there is just as much if not more child abuse. And horse abuse, for that matter. Not that abuse of women is lacking at all.)
In the end, this just expanded TBR list instead of shortening it.