The Bastard Prince. Captured Hearts, #2.5

Reyes O'Bannon lives a fairly quiet life, despite his role as the King's secretary. His days are filled with appointments, paperwork, and avoiding the romantic attentions of Kinnaird, the Duke of Keyes; attentions that become harder to ignore with each passing day... Then the Kingdom is attacked, and the nature and location of the attacks make it painfully clear there is a traitor in their midst. The attacks are further compounded by the sudden arrival of a man who claims to have a legitimate claim on the throne, a bastard prince who knows things from the King's past that no one else could know.
As the problems increase, both within the palace and across the Kingdom, Reyes can only watch as Kinnaird is sent out to unravel the mystery and the King begins to fall apart.