I was so intrigued by the first episode that when I realized it was a book, I had to have it. I didn't want to wait 6 or 8 TV weeks to find out what the story was.
I barreled through it in two evenings - it was hard to put down. The series has made some deviations from the book (the lead detective isn't weird) but so far, nothing huge. I'm glad I know what the deal was and I'm also glad I read it first. I think the back and forth timeline reads (literally) better than following it visually. The book details are more fleshed out and things like why Cora's father sleeps in the same room with her are quite clear. It was written by a German author so some of the names and all of the places are different. In the book, quite a bit happens "inside her head" regarding thoughts/feelings along the way so you get a better understanding as to her actions.
I really enjoyed this book and am curious to see if it can translate well to the screen. To provide a barometer, I wanted to break things when I finished Gone Girl and refused to see the movie.
I would not, however, describe Cora as an unreliable narrator.