Switched - Switched, #1

Wise-cracking mechanic Jessie Wyndom gets the surprise of her life when she's accidentally transported aboard an Alliance of Planets starship. Then, she learns an identical twin who was raised on an alien planet has taken her place on Earth. They are victims of a scientific experiment that separated Terran twins before birth. Jessie doesn't believe it. This is all a gag perpetrated by her brother and his nerdy friends.
Years of discipline enables Captain Marcus Viator to maintain the austere, emotionless persona prized on his planet, Serenia. Until he meets the bold, impertinent Jessie. She must go back to Earth before she disrupts his well-organized life. When he tries to return his unwilling visitor, he discovers the ship has left Earth's orbit--supposedly at his command. When more sabotage crops up, Marcus and Jessie must work together to protect the crew and themselves.
As their attraction grows, treachery and violence force them into decisions that could separate them forever. Revised version of book published in 2001.