Marielle Clarac no Yuuwaku

Book 3 of the Marielle Clarac series.
With her plain appearance and inferior social status, Marielle never expected to be engaged to Simeon, the Vice Captain of the Royal Order of Knights and heir to a highly esteemed house. And yet, he is her fiancé, not to mention everything she could ever wish for in a man. For him, she’d do anything—even feign politeness in polite society and do her best to be presentable. As she prepares for the wedding with great enthusiasm, she continues to pursue her secret profession as a romance novelist. However, things take a turn when Simeon is accused of leaking official secrets. To save him, Marielle starts investigating—and an enigmatic figure enters her life... The third volume in the adventures of the devious fangirl who secretly obsesses over her fiancé and everyone around him!
(Source: J-Novel Club)