Agent 21: Codebreaker: Book 3 by Chris Ryan
Secret agent Zak Darke is back for his third mission - and the stakes have never been higher. An unknown bomber is conducting a terror campaign in London. After an explosion on the tube leaves someone dead, Zak and his team are brought in to try and work out how this terror cell operates - but clues are scarce and they have no idea where, or when, the bomber will strike next. A teenage boy, currently detained in a young offender's institute, claims he has the answer - but before Zak can question him, the boy is shot and falls into a coma. Will Zak be able to break the cipher before the bomber strikes again?