This is a sourcebook concerned with how the police would, presumably, investigate crimes with an occult angle. I wouldn't say "supernatural", though, as it centers more on "ritualistic" aspects spotted at a crime scene, linking the crime to an occult methodology. There is also a rather simple adventure scenario ("Murder by Numbers") included which takes the Investigators through the basic procedures in collecting evidence and cross-referencing phone numbers.
It's an OK book with decent artwork. With a stronger occult angle (for instance how these police procedures regarding bites/ claw marks connect to monster taxonomy, how the investigative process deals with a possible haunting, and how witch curses may upset the cause- effect of the material world and so be identified by the inherent probability of coincidences) it would have been more useful for Horror rpg GM:s.
As for fact checking, on page 5 this book boldly asserts that "the majority of all crimes" are impressively "solved" within 24 hours. From the context it appears that "crimes" here refers to not only violent crimes, but also petty vandalism. With "solved" I would out of courtesy assume the author means "solved from a police perspective in the sense that the guilty party has been identified/apprehended", as the court procedings are likely to drag on for months (or even years) after a serial killing until a conviction is in and the case is legally solved. To a Swede, those figures are still baffling to say the least as, for instance, less than 2% of all burglaries are ever solved on this side of the Atlantic, let alone within 24 hours. However, maybe the author simply means that "Of the crimes who are ever solved, the majority is solved within 24 hours, so time is of the essence when conducting an investigation"?
Further regarding ambiguous claims in the book, I also find the assertion that police officers are trained ("courses and seminars") to distinguish killings carried out by irrational killers a bit of a stretch. As the lack of rationality would make the motive more or less moot and change the basics of an investigation that may seem like a crucial question. However, a famous professor in criminology (Leif GW Persson) once alleged that "motives are just the graving on top", and went on to liken profilers with "mind readers". According to him at least, speculation on a possible motive should never be allowed to influence the allocation of police resources or its list of suspects. Hard evidence should always determine the investigative process rather than pseudo-scientific and vain attempts to peer into the psychology of a criminal. Facts, not hunches, so to speak. At least as long you'd stick to mundane crimes.