Married by Morning — Lisa Kleypas

He is everything she wants to avoid...
For two years, Catherine Marks has been a paid companion to the Hathaway sisters -- a pleasant position, with one caveat. Her charges' older brother, Leo Hathaway, is thoroughly exasperating. Cat can hardly believe that their constant arguing could mask a mutual attraction. But when one quarrel ends in a sudden kiss, Cat is shocked at her powerful response -- and even more so when Leo proposes a dangerous liaison.

She is not at all what she seems...
Leo must marry and produce an heir within a year to save his family home. Catherine's respectable demeanor hides a secret that would utterly destroy her. But to Leo, Cat is intriguing and infernally tempting, even to a man resolved never to love again. The danger Cat tried to outrun is about to separate them forever -- unless two wary lovers can find a way to banish the shadows and give in to their desires...