I Am Number Four ( Lorien Legacies) (Hardcover) by Pittacus Lore

Two alien races use Earth as a battleground in this engaging, fast-paced young adult science fiction novel. When the planet Lorien is destroyed, nine teenagers and their guardians escape to Earth, hotly pursued by their enemies, the Mogadarians. The Lorians' legacies, their incredible powers, are blossoming, and drawing attention. Being hunted and killed in order, Number Four is up next. Known as John Smith and living in Ohio, he's reluctant to deal with the threat due to, of all things, falling in love.

In rural Ohio, friendships and a beautiful girl prove distracting to a fifteen-year-old who has hidden on Earth for ten years waiting to develop the Legacies, or powers, he will need to rejoin the other six surviving Garde members and fight the Mogadorians who destroyed their planet, Lorien.