As far as books concerning the Celts published by Llewellyn go this is one of the best. The MacCrossan has a good basic knowledge of Celtic linguistics and history but he treats the cultures as if they were things of the past. His rituals are also so dry that the ancient filidh and druídh would probably cry. I'd suggest that you pick up a copy of _the Apple Branch_ if you can find it. When entering into Celtic Paganism it's important to remember that the Celts are still posses living cultures and languages.. MacCrosan really seams to forget that.
I am only passingly familiar with MacCrossan's book, but it does appear to have certain fundamental flaws- such as dry ritual scripts, and an anachronistic focus on reconstruction rather than living Celtic tradition. But as far as I know, and according to Celtic scholars who have read the book, the basic historical information is for the most part correct. I'm posting this to balance out the previous review- anyone who considers Cunningham a reliable source cannot possibly know what they are talking about.