So This Is Ever After by F.T. Lukens

An instant New York Time bestseller! Carry On meets Arthurian legend in this subversive, “delightfully original and whimsical” (Kirkus Reviews) young adult fantasy about what happens after the chosen one wins the kingdom and has to get married to keep it…and to stay alive. Arek hadn't thought much about what would happen after he completed the prophecy that said he was destined to save the Kingdom of Ere from its evil ruler. So now that he's finally managed to (somewhat clumsily) behead the evil king (turns out magical swords yanked from bogs don't come pre-sharpened), he and his rag-tag group of quest companions are at a bit of a loss for what to do next. As a temporary safeguard, Arek's best friend and mage, Matt, convinces him to assume the throne until the true heir can be rescued from her tower. Except that she's dead. Now Arek is stuck as king, a role that comes with a magical catch: choose a spouse by your eighteenth birthday, or withe