The book is a single novel, though IIRC, it read more a collection of related "cases", with the same recurring characters. I believe the story's timeframe is late 19th to mid-20th century Britain. The police - whom IIRC, are with Scotland Yard - carry both firearms and swords (often referred to as "a smallsword"), and magic is an integral part of both their job and the crimes they solve.
I'm afraid I don't remember many details about the various cases they investigate, but I do remember one specific detail from a crime scene. The investigators are examining a house, and the subject of the dangers of magic comes up. A senior officer tells a younger one a story about someone who died as a result of a spell he concocted to rid his flat of bugs. While preparing a concoction over the fire, several drops of sweat from his forehead dripped into the pot. When the spell was enacted, the conjurer was affected by his own death spell, and he died of a fever.
IIRC, the word "magic" or "sorcery" or something similar is part of the title. I seem to recall that the cover photo was of an outstretched hand, a pentagram on the palm, and some sort of sorcerous fire coming from the fingertips.
Timeframe of Publication
I think this one came out in the 1970s or 1980s, though it could be older. I almost certain I read it in the 80s, so it would not be more recent than that.