This book is a classic, and is one of the best collections of Celtic myth on the market. Maybe _the_ best. Everyone who is a mythophile or a Celtophile should have a copy of this.
That said, it isn't perfect. Most of the flaws in it can be traced to the time in which it was written (1912). First, it stretches too far to compare everything to a Greek or Roman myth. To call the Dagda "Zeus" or Branwen "Aphrodite" is a little inaccurate, in my opinion, but I try to keep in mind the fact that he was presenting the Celtic myths to an audience obsessed with Greek myths. He even mentioned in his foreword that part of the reason for writing the book is because he was bored with poets' constant classical allusions, and wanted to give them a fresh well of legend on whicb to draw. So, in drawing parallels between Celtic and Greek myth, he was probably just trying to translate the Celtic myths into a format that his audience would understand.
The second, and more serious, gripe is Squire's anti-paganism. He buys into every rumor ever spread about Druidic human sacrifice. While at least one body has been found which was probably the victim of sacrifice, there is no evidence I've seen to indicate that the Celtic religion was the bloodbath it has sometimes been made out to be. Human sacrifice seems to have been present but very rare. Squire loves the Celts' stories, but tends to present the people themselves as bloodthirsty savages in dire need of Christianity to "civilize" them. Again, this may just be a product of Squire's times; he might have had to bash paganism just to get his book published in those days. Or the stories of widespread sacrifice may have been more commonly accepted as historical fact. I don't know. But if you can take his bias with a grain of salt, this is an excellent resource for anyone interested in Celtic myth.