Nightshade by Andrea Cremer

Calla is the very dutifully-minded, strong Alpha of the Nightshade pack, and takes her position quite seriously. She's always known she would be paired by the Keepers with Renier, the gorgeous, popular Alpha of the Bane pack. The wolves do as the Keepers say, and in return the Keepers see to it the wolves have everything they could ever possibly want. When Calla comes across a lone hiker in the woods being attacked by a bear, she is torn between letting nature take it's course (and the hiker surely dying), or saving him, which means revealing she is a wolf. She simply cannot walk away from him. Saving Shay was the first in a long line of fate-altering events for Calla, as she discovers when he shows up at her school soon after. When she is charged with befriending him by her Keeper she starts to realize their meeting in the woods was much far than coincidence. This boy poses the questions Calla has never sought to answer, and her destiny is forever altered. Nightshade was a very enjoyable, fast-paced adventure.