I recommend this book for: grade 7 and up
I read this one months ago, promptly devoured the sequel, and have been waiting impatiently for the third ever since. In this adventure full of steampunky goodness, WWI is just about to explode in Europe. Alek, the teenage son of Archduke Ferdinand, is on the run to escape those who want to kill him. Deryn, a young Scotswoman who passes as a boy to serve in the Royal Airforce, gets her dream assignment aboard the Leviathan - a living airship created from a genetically modified whale, among other creatures. These two are theoretically on opposite sides of the conflict, but they are forced to cooperate when Leviathan crashes near Alek's hideout and Deryn takes him prisoner.
A little background knowledge on WWI would help with understanding the story, but Westerfeld's world-building is so good that it's not necessary. And the history of Alek and Deryn's world is definitely different anyway - did I mention the mechanical walkers and the giant floating jellyfish? Fast-moving and fascinating, I would recommend this one to a wide variety of readers.
But don't take my word for it - check out the review and excerpt!
And if you like this one, try: Airborn by Kenneth Oppel