Under Cover by Chris Ryan
Ricky is a street kid - sharp, quick and usually up to no good. When he tries to steal from the wrong people, he is saved by a mysterious man called Felix who makes Ricky a compelling offer: a flat and a hundred pounds a week. All he has to do is take lessons from Felix.
Soon Ricky finds himself learning about surveillance techniques, how to make himself invisible in a crowd and hand to hand combat. But what is this all for? Ricky has no idea until he's given his first mission and finds his whole world turned upside down.