Truly, Madly, Deeply by Jeannie Moon

Nick DeMarco was a covert operative who breezed in and out of the lives of the people he cared about, often without warning. This included his sister Josie’s best friend, Lila Novak.
Nick and Lila had a torrid affair the last time he dropped into Barefoot Bay, and memories of her kept him going when his last mission went bad. Now that he’s back, possibly for the long haul, he hopes Lila will forgive him for disappearing without a word.

Lila loves her life as a teacher at Mimosa High School, but her affair with Nick has thrown her a curveball she never expected. Carrying his baby, she’s faced with the loss of her job unless she agrees to to do the last thing she wants to do – marry him.