A Spy at Pemberley — Fenella Miller

A Spy at Pemberley is the third and final book in the 'At Pemberley' series. Darcy and Lizzy have been married for some time and their union is going through troubled times. This is bought to a head when Caroline Bingley, who always had designs on Darcy, arrives unexpectedly.
If having Darcy's old flame under her roof wasn't enough for Lizzy to contend with Colonel Fitzwilliam, an intelligence officer, arrives also. However, this isn't a purely social call as he needs Darcy's assistance to entrap two spies who are passing secrets to the French. Against her better judgment Lizzy is drawn into this dangerous escapade and asked to invite the suspects to a house party at Pemberley.
As Lizzy and Darcy dash from Pemberley to London and back again they not only unmask the traitors but rekindle the spark that brought them together all those summers ago.