Readers of Chris Ryan thrillers know exactly what to expect: gritty, pared-down prose with regular doses of bone-crunching action and a hero not usually given to introspection. His books do not inhabit the same atmospheric world as such past thriller writers as Graham Greene and Eric Ambler, and he lacks the political sophistication of current ones like Gerald Seymour. But Ryan knows what his readers want and can always be counted upon to deliver a tough and fast-moving package. The theme here is a duel to the death between an SAS soldier and the man who trained him. Ryan's protagonist, Alex, has been recently commissioned from the ranks and returns from a hostage rescue mission in Sierra Leone to find someone has been gruesomely murdering MI5 officers, Hannibal Lecter-style (a skinning knife is involved). The security services believe the killer is an insider, SAS-trained, and Alex is ordered to track him down. As he gets closer and closer to the eponymous Watchman, the body count rises implacably. This one will glean no literary prizes, but bookshop tills will ring as merrily as they did for The Hit List and Zero Option.