From the very first page, when we are told that the city of London has gone hunting, I was completely hooked on the world Reeve created. Mortal Engines is set in a steampunk, post-nuclear world. Cities are on wheels, most of them, and consume each other in a system called Municipal Darwinism. Survival of the fittest city. The problem with that should be immediately obvious: sooner or later there will be no other cities to consume. And that's the central conflict here, how and whether London will continue as a moving city.
There's also fantastic viewpoint characters: Tom, who gets knocked off the city and learns to understand it better on the ground, and Kate, who is determined to learn the truth about her father and finds more than she intended. And then there's the supporting characters, sweet and brave Bevin and Hester, who is made of determination. And a villain who ends up being far more than he had seemed.
The ending is explosive and keeps going right up to the last sentence. There's hardly time to catch your breath before you realize that it's over, and you need to read the next book ASAP so you can find out what happens.