Fun and with a fine theme, but includes some body humour.
Plot
Hiccup returns in a tale of fiendish Romans, miniscule dragons, a crazy sword-wielding girl, and more. And to think it started with a pirating lesson.
Morality
Hiccup and Fishlegs are faithful friends and even the stuck-up Toothless pitches in. Hiccup manages to befriend a suspicious girl and his wisdom (eventually) gets through to his father. Bad guys are clearly defined.
Spiritual Content
Mentions of Valhalla, Woden, and Thor. The Romans worship Jupiter and have temples to their gods set up. One dragon proudly calls himself "the Living God" a couple times. At one point, Hiccup is described as "godlike" and claims to be Thor.
Violence
One man eats little dragons live. The Romans capture dragons for food, their hides, and "entertainment" in the coliseum. Birds and small dragons are killed and eaten in the Colosseum. Dragons attack people (no casualties). Fishlegs is nearly killed. Hiccup barely escapes being dragon-bait. A man booby-traps an item with a poisonous dragon.
Drug and Alcohol Content
The Romans can buy food and wine before Colosseum entertainment.
Sexual Content
There's a reason why "Big-Boobied Bertha" has the name she does. Attention is called to them in an illustration and in the fact that her size has aided in her hunting and killing. At one point, Fishleg's pants fall down to his ankles in front of a girl, but nothing comes of it.
Crude or Profane Language or Content
Hiccup, Fishlegs, and Toothless are called various names (most of these are from one rather imperial dragon toward Hiccup). Body humor continues more on a "How to be a Pirate" scale. A man at one point purposely vomits so he can eat more.
Conclusion
This book is as crazy as its insanely fat Roman consul and wild warrior girl, but a fun adventure including - but not limited to - childish Viking temper tantrums, incredibly proud dragons, and the ingenuity of the children - with a great theme that "size isn't everything" to wrap it up.