Marielle Clarac no Seikan

Book 7 of the Marielle Clarac series.
Marielle and Simeon have finally settled into newlywed life. While author Marielle may seem plain and royal guard Simeon may look villainous, actually they’re anything but—not to mention perfect for each other! When Marielle travels to a distant region of Lagrange to attend a funeral, she’s surprised to see Simeon arriving on a secret mission with Prince Severin in tow. What’s going on? With war brewing, Marielle soon finds herself embroiled in another high-stakes international incident, as old friends and enemies return in a tale involving foreign royalty and a hunt for a sacred treasure. The seventh volume in the adventures of the now happily married fangirl who obsesses over her husband and everyone around him!
(Source: J-Novel Club)