Shameless

Lady Elizabeth Banning, the youngest and most headstrong of the three Banning sisters, is a shameless, hardened flirt with a reputation for jilting fiancÉs. Perhaps she is reluctant to marry due to her father's appalling treatment of his wives and daughters, or perhaps because she's headstrong and likes to do as she pleases. When a mysterious man helps extricate Beth from a potentially scandalous situation that arises after she throws over another fiancÉ, Beth doesn't realise she has become a weapon government-sanctioned assassin Neil Severin can use against her brother-in-law Hugh, the Duke of Richmond. A member of the War Office who has determined that Neil is a menace to society and must be taken out, Hugh is the only person who knows Neil's true identity as the Marquis of Durham. If Neil kills Hugh before Hugh and the War Office kill him, he can re-assume his identity as a nobleman with no one the wiser about his bloody but patriotic past.
When Neil attempts to abduct Beth in order to force a confrontation with Hugh, one of her angry jilted fiancÉs beats him to it. As Neil rescues Beth from a medieval fortress on the Scottish border where depraved aristocrats purchase doxies, he and Beth weather many adventures and fall in love. Clever Beth comes up with a way of negating the official death sentence against Neil.