The Heidelberg Effect (The Tempus Fugitives Trilogy) (Volume 1)

Ella Steven's stubborn independence is the keystone of her identity--unfettered by romantic involvements or family demands--until the day she takes an exciting new job in Heidelberg, Germany. There, she stumbles onto a hidden time portal that takes her to 1620 Heidelberg where all her modern-day techno toys and proud self-reliance can't protect her from the brutal realities of every day life.
 
Befriended by a convent of seventeenth century nuns who stand on the executioner's block of the bloodiest warlord in all of Europe, Ella struggles to survive in this primitive and brutal time. With the help of a sexy US Marshal, she tries to break out of her closed world of protected autonomy to help her new friends. When she does, she learns the hard way that when it comes to the things that really matter in life--love, trust and friendship--sometimes opening yourself up to others is the only true way home.