Contains spoilers
First things first: before I start reviewing this book, I recommend you read book two in case you are not reading this series in order. Just because, at least for me, books two and three are two parts of the same arc, and book three gives some resolution on some stuff that happened in book two.
Now, about book three. As always, we are presented with new and interesting dragons, along with some other characters. Hiccup and his friends are thrown into an impossible situation where he must use all his cleverness to take them out. From here, there is nothing too different from what we saw in the other two books; the interesting part came in the way Hiccup finds the solution to the final riddle.
Before I dive into that, I need to make some clarifications. Spoilers ahead. If you have already read book two, you know the arc of Alvin the Treacherous was the main plot of book two. Yes, the story presents the Strangler as the final boss, but in reality, the plot of the book ends inconclusively as the author leaves hints that we haven´t seen the last of Alvin. The arc we have been following from book two is resolved in this book, as we are given a most definitive final for Alvin, who is the main antagonist in book three. Of course we get the big bad dragon as it is the hallmark on HTTYD series, but the big bad boss is ultimately Alvin.
Since book one Hiccup story have shown us that there are more than one way to be a hero, and that most times than not what we need is the most improbable and desperate plan. In book three, Hiccup must learn that sometimes help came from the most unexpected places.
Contains spoilers
Not bad, but not as good as the first one.
This continuation of Hiccup´s story is not a bad book, but it is not as compelling as the first one. Most of all, in the end, it is impossible not to feel that the story falls short. While in the first book the author makes a great presentation of Hiccup´s world and all the characters who surround him, in this book, the presentation of the big, bad boss that Hiccup has to ultimately defeat lacks the weight it should have because it is thrown at us with little to no previous context.
Another thing is the resolution of the conflict. I have always thought that just because you are reading a children's book, it does not mean the book should treat its readers as stupid. Like any other HTTYD book, the big final boss is a dragon, but in this case, it does not feel like the great menace it should be; instead, it feels more like a poor resource of the author to end the story. This has a lot to do with the fact that this novel works more like an introduction to Alvin's character, who will have a more preponderant weight in the next book. And yes, Alvin is by great means the enabler of this book's plot, but the introduction of the final boss felt as if the author remembered she had to put a big bad dragon Hiccup has to defeat, and she threw him in the last two chapters hoping the readers would not notice. Even the way Hiccup defeats it feels waaaay too easy. Comparing it to the finale of the first book, where Hiccup has to use all his wittiness and cleverness to solve the riddle, in this book, the answer is given to easily and too in-the-face, we don´t get to have that moment where we are surprised by Hiccup's abilities. Instead, we know how the problem is solved even before the dragon had shown us he is poisonous. The moral of the story is not less beautiful or interesting than the last one, but the way we get to that conclusion do not make it justice.
Considering that this book series was not thought of as a saga per se, but more of a collection of stand-alones, I will say that if I had read this book as my first impression of the How to Train Your Dragon books, I´m not sure I would read the rest of it.
First things first, if you came to this book after the movies, you need to know these two are different stories, so don't expect a novelized version of the movie. Yes, they share some characters, such as Hiccup, Stoic The Vast, Fishlegs, and of course Thoothless. But that's where the similarities end. Think of this as Resident Evil the game, and Resident Evil the movies. They share part of the lore, but they are primarily different stories.
That being said, the story of How to Train Your Dragon is a great story about how to become a Hero the hard way. You need to remember this is a children's book, so yeah, many parts of the plot will be solved with the power of friendship. That doesn't make the story less relevant; in the end, everything Hiccup shows throughout the chapters is that sometimes you need to embrace all those quirks that make you different.
Whether you are reading it as an adult, or reading it for a child, this book has something to offer if your willing to play the game of imagination.