Starclimber by Kenneth Oppel

From the back of the book:
At long last, Matt Cruse is at the helm. Though it's only a summer job piloting a humble aerocrane, he's thrilled to be a small part of something big. With every load, he's helping to build the Celestial Tower, Paris's extraordinary gateway to outer space. But Matt's idyllic summer is short-lived. He narrowly survives a deadly attack by the fanatical Babelites, who are opposed to human beings reaching the heavens. Worse still, his nights spent stargazing with Kate de Vries end when her parents summon her back to Lionsgate City. It's time she began to think about getting married. But then a chance of a lifetime boosts Matt's hopes of being airborne once more. Canada wants to reach space first, and the Canadian Minister of Air has asked Kate to join the first expedition as an expert on aerial zoology. There's a place for Matt, too - if he can pass the gruelling tests to become one of the world's first astralnauts on board the incredible ship, Starclimber.