Aedion Ashryver from Throne of Glass doors by Sarah J Maas.

You are a spy working under Aedion, currently working out of Fenharrow, a lawless area near the southern border of Erilea, close to Terrasen. Your letters back and forth have become increasingly flirty and affectionate, leading to the General's most recent letter... Will you surprise him with a visit, or another bold letter?