In Bed With the Boss — Susan Napier

From model secretary...
For three years, Kalera Martin has been a model secretary to Duncan Royal, with only one hiccup in their otherwise perfect relationship: a one-night stand that should never have happened, and which both have tried to forget. Or so Kalera thought.

...to the boss's mistress?
But Duncan is haunted by their one night of unbelievable passion, and secretly longs for their relationship to develop after office hours. As a rule, he can have any woman he wants -- so he's furious when Kalera announces her engagement to another man! Whatever it takes, Duncan intends to entice her into his bed once more -- and this time it will be forever.