The Black Lily - The Arestea Chronicles, #1

Courtesan. Spy. Assassin. Across the Kingdom of Arestea, the shadowy league of professional killers known simply as the Guild has long since earned its terrifying reputation. And none of its current members are more infamous than the Black Lily. No one knows who the Lily is, but everyone recognizes the efficiency with which he or she brings down even the most guarded targets. There is no one, it is said, who is safe from this fiend once they have caught the assassin's attention.
Now Lily herself is about to discover if her reputation has been inflated or not, for she has just been assigned the most daunting mission of her career: infiltrate the royal palace and eliminate the entire Arestean line of succession to make room for the Guild's puppet ruler. It's a challenging job, but one that will secure her place in history should she succeed. But when unplanned circumstances take the king from his country to help secure the front lines in his latest war of expansion, Lily is left trapped in her assumed persona behind the palace walls and forced to stall for time.
And when a particularly bad stroke of luck reveals her cover to the king's brother, Crown Prince Adrian, Lily finds herself ensnared in her own web, forced to use all her skills of subterfuge and manipulation if she is to stay one step ahead of the naïve but righteous young man and finish her mission - or die trying.