The Final Silence by Stuart Neville

DI Jack Lennon and DCI Serena Flanagan must join forces to investigate a series of murders reaching back decades. Rea Carlisle inherits a house from an uncle she never knew – and with it a leather-bound book containing fingernails, locks of hair and a list of victims. Horrified, Rea turns to the only person she can think of: old flame DI Jack Lennon. But Lennon has his own troubles, and they only get worse when a brutal murder places him in the crosshairs of one of the force’s toughest detectives: DCI Serena Flanagan.

Lennon soon realises that running isn’t an option, and an alliance with Flanagan is the only way to find a killer with nothing left to lose.