So far so what, except that Caleb Carr's debut novel is set not in a contemporary city, but the fledgling fin de siecle New York of a hundred years past. Forensic methods the police now rely upon (and a few they don't - people are still photographing the eyeballs of corpses, probing for that elusive image of a killer) are in their infancy.

At the behest of Police Commissioner Theodore Roosevelt, who pushes aside his own doubts about "mystical mumbo-jumbo", into the fray steps the enigmatic Laszlo Kreizler. In a time when the mentally ill are "alienated" from society, those, like Kreizler, who study them are deemed Alienists. Thus the neophyte psychologist assembles about him a team of detectives, former patients, and narrating journalist John Moore on this adventure to trust to science to trap a madman.

There is no denying the amount of work Carr's put in here. The early pages brim to overflowing with the often squalid urban detail of a city on the turn of a century - police no-go areas, overzealous clean-up gangs, backwoods drinking dens, a vivid and volatile racial mix, the gigantic reservoir built in the heart of the city, police and church corruption. It's all good, solid stuff, and not a little stodgy into the bargain. Carr is forgetting that authenticity in a thriller is no substitute for thrills. Fortunately a third of the way through his hefty (read: excessively long) tome, just when it seems like he's thrown it all away, Carr suddenly forgets all about his text books and an pleasingly intricate, rather inconceivable plot makes a mad dash at the distant finishing-line.

The book opens round at Thomas Harris' house with an extraordinarily grisly mutilation-murder and the gaol interview of a convicted killer, as though the author is anxious to get comparisons out of the way early. It's a good move. Certainly thereafter this reads less like Silence of the Lambs and more like one those huge doorstop-thrillers much beloved of child psychologist Jonathan Kellerman as the investigative team use the new science of daguerreotyping (fingerprints to you) and a primitive form of psychological profiling to track their man in the crowd.

There's a satisfyingly lurid, gruesome grasp of the sticky details (a jar of eyeballs, preserved human hearts, brain dissections) but Carr's grasp of characterisation is at best shaky. These people are little more than puppets, stereotypes (especially Sara, the plucky young woman who dreams of becoming the city's first female detective) dancing to a creator's tune. There's no sense of life outside of these pages. As a consequence the book can seem at times flat save for the mystery itself. But the mystery - the chase - is the thing, and Carr has an lively instinct for plotting and the sense to draw all of this together in a gratifyingly energetic climax. It's hardly Thomas Harris, but until homicide's reluctant superstar deems to put another public appearance, The Alienist will do to be getting on with.