I'm afraid I came a little late to this party. I've just discovered I've read #5 and now #7 (this one) in a series. I hate doing that! Sometimes you never read anything that'll give the game away. Sometimes it's the fault of the fault of English translations (Jo Nesbo's Harry Hole's first case is much referenced in later books, but was only available later). Anyway I digress.
Detective Cassie Maddox agrees to go undercover to find the murderer of a woman who is her doppelgänger. She has to seamlessly return, believed to have survived a stabbing, back to her four housemates.
This novel is basically a story of an undercover cop, who you don't actually know will or wants to return to real life. She lost her parents at a young age and identifies with her new housemates and their "no pasts" mantra. She finds herself slipping into their closeness. I found the theory that someone could fool others into believing they were someone known to them slightly incredible, but then you worry she is going to get burned every day ... The story is tense and a definite page turner. In fact I stayed in bed this whole morning to finish it!