THE York writer of time-slip thrillers zigzags between modern-day Kosovo, Rome, London and York, and the era of the Emperor Constantine.

Abby Cormac, a former investigator for the International Criminal Court, witnesses the murder in Kosovo of her diplomat lover, a man more mysterious and dangerous in death than in life. Shot and shaken up, Abby tries to piece together what happened. Her story then runs alongside an investigation into a murder committed in Constantinople in the year 337.

Harper certainly knows how to construct a parallel plot and his two stories chase each other along as the novel rushes to a wonderfully dramatic conclusion, with the present-day mystery nicely sheathed in a secret hidden for centuries.

Lies ancient and modern engage in a breathless whirl, and a relatively weighty book whizzes by, thanks to robust but understated writing and a succession of cliffhangers: when the modern story stops with its toes on the precipice, the narrative jumps back to the ancient tale, and vice versa.

The historical sections wear their learning lightly, with only the names causing momentary puzzlement: who was that again? But this remains an intelligent thriller that delivers the goods, while also imparting a history lesson.