This is a supremely well crafted psychological thriller, with imperfect, believable characters who do imperfect, believable things. French's prose is effortless and evocative. Her modern Ireland -- torn between its prosperous present and its downtrodden past -- is a character as fascinating as her protagonists.
I found In the Woods hard to put down and would have read it in one sitting if I had had the time. I do not read many books in this genre, but this is almost certainly the best of its sort I have read in a long while and the best book I have read recently.
Fortunately for me, Tana French has written two more novels with some of the same characters, and they are next on my list. These are The Likeness and Faithful Place.
You may wish to avoid this if you are bothered by graphic crime scene descriptions or explicit language.
-Paul
If you like Tana French, give the following authors a try:
Benjamin Black (beginning with Christine Falls)
Elizabeth George (try Traitor to Memory)