Insidious is the 20th book in Catherine Coulter’s FBI Thriller Series and tackles the common two case arcs of FBI thrillers prevalent in these types of books today. One set of married agents embarks on a game of “Clue” like whodunit to determine the culprit behind a murder plot against one of the most accomplished business women in America. The other agent works with local police on a search for a serial killer attacking young female Hollywood hopefuls. While the police procedural elements, sleuthing and evidence gathering have an air of authenticity; the overall plot suffers from a lack of suspense — a thriller devoid of thrills. The characters need more development to capture your imagination and make you care about the danger they face. The reveal of the culprits seems rushed at the end and is ultimately unsatisfying. While Coulter’s writing style makes the book an easy read, the lack of a real sense of danger and flat characters make this book just as easy to put down.