The Abbess of Vlaye by Stanley J. Weyman

It is possible that Bonne did not herself know in what proportions pity and a warmer sentiment entered into her motives when she undertook to pass for the Countess and assume the girl's risks. Certainly her first thought was for the Countess; and, for the rest, she felt herself cleared from the reproach of unmaidenliness by the danger of the step which she was taking. Even so, as she rode across the camp in the dusk of the first evening, into the half pain, half pleasure that burned her cheeks under the disguising hood entered some heat of shame.

Not that it formed a part of her plan that des Ageaux should discover her. To be near him unknown, to share his peril whom she loved, while he remained unwitting, to give and take nothing -- this was the essence of the mystery that charmed her fancy, this was the heart of the adventure on which her affection had settled. He, by whose side she rode, and near whom she must pass the dark hours in a solitude which only love could rob of its terrors, must never know what she had done for love of him; or know it only from her lips in a delicious future on which reason forbade her to count.