Dark Places is one of Gillian Flynn's earlier books (by "earlier," I mean before this summer's ubiquitous Gone Girl, which I liked but did not love). I happen to remember Flynn from Entertainment Weekly, which is my favorite magazine.
Dark Places follows Libby Day, and adult whose mother and sisters were murdered when she was a child. Libby testified that her brother, Ben, was the killer, and he's been in jail for the 25 years since. Libby has been drifting and living off donations, which are almost out. So when Lyle and the Kill Club (a local group of people obsessed with various murders and murderers) contact her and offer to pay her for helping them solve the crime (they believe Ben to be innocent), she takes them up on it for the money.
And to reveal too much more of the plot would be spoiling it. I can say that the narration shifts between Libby in the present day, Ben the day of the murders, and mom Patty the day of the murders. It's a stressful day for a lot of reasons (family is poor, Ben is a teenager, several pieces of bad news are delivered etc.). By the end of the book, Libby, the Kill Club and the reader all know true story of the murders.
Did I like it? The action was well-paced, the dialog is believable, and the story was enjoyable. However, I thought the ending was a little too tidy. A quick scan of Internet reviews show that I am in the minority here, but I though the deus ex machina was a little too unbelievable. So, my final verdict is that it's well-written but ultimately I wouldn't recommend it.
Did it pass the Bechdel test? Yes. Libby doesn't necessarily interact with many women (she's a loner), but Patty interacts with her sister Diane, and childhood Libby and her sisters Debby and Michelle talk amongst each other a lot.