So I took out every volume of Dungeon that the library stocks. Now, you have to understand that Dungeon has several incarnations - Zenith is the main story. The Early Years is the ancient past. Parade are humorous side stories that take place during Zenith. Downfall is the era after Zenith, and Monstres tells epic adventures of side characters in Zenith. Each separate arc is two or three books long, and the library doesn't have ANY sequential books! Not even a single two-book sequence! So I'm reading a completely scattershot selection of Dungeon books.
The Barbarian Princess finds the central cast, notably Herbert (the irresponsible duck) and Marvin (the brusque dragon), trying to boost tourist traffic at the Dungeon by claiming that there is a princess being held captive in the area. Turns out Herbert accidentally names a real princess from a real kingdom, and she's missing, so the Dungeon winds up being threatened with war by her father! The princess, Isis, is hilarious after she's introduced, and she winds up accompanying the heroes on a great quest (which partly involves finding her a wedding dress, a great twist unto itself!). Trondheim and Sfar keep enough twists coming, and keep things just serious enough that you care how the story resolves, but they clearly have a lot of fun along the way.