A Lady’s Handbook of Espionage by Katrina Kendrick

He is Her Majesty’s most lethal weapon.
Ronan Callahan shed his criminal past to rise through the ranks as the Crown’s deadliest spy. But for years, he’s always been a step behind the elusive thief known as Spectre, a cunning woman who has led him on a cat-and-mouse chase across the globe.

She is the underworld’s most wanted mastermind.
Isabel Dumont clawed her way from the gutters of Paris to become the Syndicate’s most skilled operative. But when she betrays the very criminal empire that forged her, she’s forced to ally with the spy she once outwitted.

Thrust into an uneasy partnership, these rival operatives plunge into an undercover mission to dismantle the Syndicate. As bodies pile up and the passion ignites, they navigate a world where loyalty is a death sentence and trust is a fatal weakness — and the price of failure is paid in blood.