A Breach In Time - Pamela Todd-Hunter

Alixandra Evans has spent most of her life studying and romanticizing history. But when an ancient brooch transports her back to twelfth-century France, she soon discovers that history is vastly different and much harsher than her scholarly books have led her to believe. 

Richard, Duke of Aquitaine and the future King of England, has his hands full protecting his duchy and keeping his troublesome nobles in line. Yet, when a beautiful young woman stumbles out of the forest alone, he can't simply leave her to her fate. Unable to resist her spirit and bravado, he offers her a position in his household, even though he senses she's harboring secrets. Alix's only choice is to accept Richard's protection until she can find a way back to her own time. Complications soon arise as she finds herself, and her heart, drawn to the man behind the legend. 

But the twelfth-century isn't Alix's time. If she stays, her presence in Richard's life could have far-reaching consequences. One misstep might set in motion events which would alter not only Richard's destiny but also the future of England. Does she dare remain with him and risk destroying the world as she knows it, or return to the twenty-first century and leave her one true love in the past?