My rating may confuse some and I don't want it to. This is a well-researched, well-written tale with a solid plot and strong characters. I particularly liked that there were strong female characters, and a real sense of sexual equality in the story. But I was overwhelmed by the subtle and not-at-all-subtle underlying message of Scottish patriotism, and the wealth of contributions of the Celts to European [or world] civilization, that the author would have you believe.
If you're of Celtic descent, or are a firm believer in Scottish independence, you will love this book. I'm neither, and I found all that reference to be tedious. I had to fight my way through what is actually a good book otherwise.
All that hype ruined it for me. Obviously, the author is trying to get a message across. That's OK, but it isn't one that particularly interests me. If you're English (I'm not), you'll probably take offense at this book - and the writer wants you to, I think.
So, unless you fit the "target" audience, don't bother with this one.