The Crystal Heart by Katherine Deauxville
Emmeline, the wife to a powerful guild master, needs to provide her aging husband with an heir. Forced to make harsh decisions, she pays her servant to go out and to pay a man for a night of 'service' that will leave her with the child she needs to give her husband. She knows nothing about the nameless man who takes up her offer. For coin, he will give her what she needs. However, instead of paying the man and them parting with little thought to what happened, she gets a night of passion that haunts her forever. She gets the child she bought...only life demands a higher payment ten years later.
Niall fitzJulien is the new overlord of Castle Morlaix. To his shock, the woman who stands before him to pay homage holds the hand of his son--his ten-year-old son. He recalls that night a decade ago, when he'd awoken the next morning from a night of passion only to discover the woman had slipped away from his bed...and taken his heart. Now he sees how cruelly he was used, how he has been denied his son. No longer. Now she is his vassal, she serves him.
A dark, strong tale of a Medieval woman who does what she must to survive, of the man she came to love, the man she must now face with her deceit.