This book has problems that made it nowhere near as worthwhile as I'd hoped:
First, it's full of fragments, except where the wholes are very short, anyway. Excerpts just make me want to see the full thing, to get the context and story properly. Second, there's poetry in here, but it's translated as prose. Whenever someone says, "It's not possible to translate poetry," they really mean, "I'm not up to the task but my ego won't allow me to admit it."
One does get a flavour of the literatures (all six Celtic languages) but it forever left me wanting more or better.