Book 1 of the Marielle Clarac series.

As a plain, unassuming noble’s daughter, Marielle doesn’t stand out in high society. Imagine her surprise when she receives a marriage proposal from the sought-after Simeon, second-in-command of the Royal Order of Knights! Simeon, the heir to an earldom, is highly accomplished in scholarly and military arts—and he’s gorgeous. So why did he propose to such an unremarkable noblewoman? Despite being a target of envy and scorn, Marielle is happy. After all, Simeon seems to be exactly her type: a dashingly handsome man with glasses who appears gentle on the outside, but who has a dark and scheming side! Welcome to Marielle’s world, where people aren’t always what they seem—and a prim, proper exterior can hide the heart of a sly and devious fangirl!

(Source: J-Novel Club)