We liken reading books to hiking mountains, and Dark Places isn't a physically demanding walk. It's like walking up a big hill: just strenuous enough to break a small sweat. Of course, this isn't a hill you're going to want to spread a blanket on and have a picnic at the top of, because you're going to have to climb over a lot of dead bodies along the way.
The most difficult part of reading Dark Places is the gruesome subject matter. A woman and two kids under ten are brutally murdered, and their killings are rehashed in graphic detail from a couple of different perspectives—and we're not talking about the distant points of view of cops or other professionals, either, but actually the perspectives of other family members. A daughter thinks of her mother's murder; a son watches his little sister get killed. So don't even bother to bring a picnic lunch—you won't have an appetite after this one.