There are books that you loved as a kid which, when you read them again years later as (at least technically) an adult, don't really hold up. Whether they simply skew too young or rely too much on familiar characters and situations because children don't have the breadth of reading experience that adults do, novels aimed at younger readers can be frustrating for older people. The best children's books are the ones which manage to be accessible enough for young readers but deep enough for adults to get their teeth into as well - books which can truly be described as suitable for all ages.
Considering that the first twist comes in the first few chapters, it's hard to say much about the plot without giving it away. But like many great books for younger readers, it involves teenage protagonists attempting to navigate a very grown-up world, with the inevitable clash of ideologies and worldviews that entails. The heroes naturally see everything in black and white, right or wrong: one of the reasons why teenage characters work so well in fiction is because they allow the author to write in these terms, and because everything is a Big Deal when you're a teenager. The slightest thing going wrong can feel like the end of the world when you're 15, so when you have 15-year-olds dealing with an actual end-of-the-world scenario, it works really well.
But Reeve's story doesn't fall into the trap of being overly simplistic that could result from this. Certainly, the heroes see the world in their own uncomplicated way, but it's clear right from the start that they're very naive about it. It's a book which constantly forces the reader to reassess what they think about the world and the people in it - allegiances are always shifting and it's never easy to tell who the "good guys" are outside the heroes' bubble of moral certainty. Frankly, there probably aren't any: with the exception of the character who ultimately emerges as the villain, no one in this story is simply good or bad, and that extends to the protagonists.
Moreover, it's a brilliant adventure story that's captivating from page one and takes a serious effort of will to put down once you're into the final stretch. The first half is fairly episodic, moving from place to place to build the world and set the scene; conversely, the second half is nothing but payoff, where the pace suddenly skyrockets, every chapter ends on a cliffhanger and you just have to keep reading because you can't bear not knowing what happens next. It's superbly constructed, and the care that's gone into building everything up and then tearing it down is in a class you don't often see in young adult fiction.
Mortal Engines is the best kind of young readers' book. It never talks down to its audience and, while remaining accessible to a reader of any age, treats everyone reading it like an adult and respects them accordingly. The end result is a book which children will enjoy precisely because they're being respected, and which adults will enjoy because simply categorising it as a children's book would be doing it a huge disservice.