Book 2 of the Marielle Clarac series.

Plain, unassuming Marielle never expected to be engaged to someone like Simeon. After all, he's Vice Captain of the Royal Order of Knights, and the heir to a noble family far more esteemed than her own. But who cares about petty things like differences of rank in high society? In her eyes, Simeon embodies everything her fangirl heart desires. He's exactly her type: the dashingly handsome, bespectacled man with a dark side. Marielle would love nothing more than to spend all her time enjoying lovey-dovey bliss with her fiancé—when she’s not pursuing her secret profession as a romance novelist. Instead she's been roped into helping the crown prince find a suitable bride. Soon there’s engagement drama all over the place—and how can Marielle enjoy her own engagement when she’s knee-deep in sorting out everybody else’s!? The second volume in the adventures of the devious fangirl who secretly obsesses over her fiancé and everyone around him!

(Source: J-Novel Club)