Tracers by J J Howard

Cam is a New York City bike messenger with no family and some dangerous debts. While on a job a chance meeting with the mysterious Nikki sends him head first into the intense world of parkour. Cam joins the group of underground teens who have turned New York City into their own personal playground- running, jumping and flying through the city like an urban obstacle course. The group offer Cam the chance to make some extra money... and the opportunity to earn some cash, and being close to Nikki, makes it impossible to say no. But Nikki is entangled with the brazen leader, and when the stakes become life-or-death, Cam is torn between following his heart and sacrificing everything to pay off his debts.