The first thing to mention is the old adage: Don't judge a book by it's cover. Seriously. This is not another Paranormal Romance. This is something very, very special.
The second thing to mention is that for more eloquent reviews, simply google for them, particularly the review by writer and lovecraftian scholar S T Joshi. These people are far more articulate than myself so I shall let them do their job and I shall be brief and, hopefully, helpful.
In my opinion Ms Kiernan is one of the finest writers of weird/speculative fiction currently living and The Red Tree finds her yet again doing the impossible; improving. The novel is written as a journal received by an agent and as such has a most distinctive voice of Sarah Crowe. It details... well, what does it detail? A descent into a terrifying insanity? The workings of deep time and old lore upon a land until the land becomes a voice of its own with the Red Tree its baleful glare? A tragic tale of a woman reaching the end of her road? All or none of the above, woven with some autobiographical textures from Ms Kiernan herself. To say the Red Tree is multi-layered is as gentle an understatement as saying it is a remarkable piece of writing. It casts shadows of the quality of Bradbury and Lovecraft and Blackwood to name a few, but always with a most distinct and unique voice. It is a work concerned with the darkest folklore in those remote counties and yet a very, very modern setting somehow that seems to bring the sense of time even more to the fore. The twists time puts on tales simply one of the layers it explores.
If you have never read any of her work, this is where you start. If I haven't convinced you, as I say seek out proper reviewers online, but mostly: treat yourself to a dark, wonderful journey.
Oh, and please read the preface....