The Twisted Sword by Winston Graham

The eleventh novel in the legendary Poldark saga. Cornwall, 1815: Demelza sees a horseman riding down the valley and senses disruption to the domestic contentment she has fought so hard to achieve. For Ross has little option but to accept the summons - and travel to Paris with his family, as an "observer" of the French armed forces. Parisian life begins well with an exhilarating round of balls and parties. But the return of Napoleon brings separation, distrust and danger to the Poldarks... and always for Demelza, there is the shadow of the secret she does not even share with Ross.