The Crystal Heart — Katherine Deauxville

IT WAS A TIME OF POMP AND PAGEANTRY, ILLICIT DESIRE...
Emmeline gave herself to a sensual stranger in a night aglow with candlelight and mystery. Then she sent him away. Wed by arrangement, Emmeline desperately needed to provide her aged husband with an heir. But her lover awakened a passion she kept secret in her heart...until he returned and rocked her world with his demands and his desire.

...AND SECRET LOVE
Niall fitzJulien had been a wild, hellraising knight when he entered a lady's bedchamber for a secret rendezvous that left him with burning dreams he couldn't forget. Now, ten years later, King Henry II has awarded him an impoverished earldom on the war-torn Welsh border. Waiting there is his destiny: the exquisite widow who possesses the riches he needs, the son he yearns for...and the passion he hungers to reclaim.

Heroine: Emmeline of Neufmarche