Hell Fire (Inspector Sejer Mysteries) by Karin Fossum

A gruesome tableau awaits Inspector Konrad Sejer in the oppressive summer heat: a woman and a young boy lay dead in a pool of blood near a dank camper. The details of the deaths of Bonnie Hayden and her five-year-old son Simon are mysterious. There is no sign of robbery or assault. Who would brutally stab a defenseless woman and her child? Sejer and his fellow investigator Jakob Skarre begin a hunt for the killer that will eventually lead them to a heartbreaking conclusion.   In a parallel storyline, masterfully fused, Fossum tells the story of Mass Malthe and her troubled son Eddie as they navigate a relationship that some would call too close. Eddie constantly thinks about his unknown father — someone his mother would rather forget. When long-held secrets are revealed, it turns out that Mass and Bonnie share more in common than Eddie could have ever guessed.