Intensity by Dean Koontz.
Horror books always start by lulling you into a false sense of comfort, and Intensity is no exception. Chyna Shepherd has just been welcomed into her friend’s family’s Napa Valley Home. It is cozy and wonderful and she is looking forward to spending time relaxing with her friend. Then the horrifically awful happens, and Chyna becomes the only surviving person of a homicidal maniac. But she can’t just hide and wait it out, because she finds out that said homicidal maniac has set his sights on another victim, and she realizes she has to do something to stop him. The book takes place in just about 24 hours, making it quite (you guessed it) intense. The killer is very creepy (one scene that stuck with me since I read this book in high school is where the killer watches a spider descend from his web and eats it, to gain its power, if my memory serves me correctly), and the tension builds to a very satisfying conclusion. It’s just that somewhere between the beginning and end it was like staring straight into scary Bambi’s dad’s face for a moment too long, and having to see his face raging at me when I turned off the lights.