The Watchman (Gekürzt‪)‬

Alex is a 36-year-old SAS Captain. Recently commissioned from the ranks, he is returning from a hostage-rescue mission in Sierra Leone when he finds himself summoned back to the UK. Someone, it seems, has been murdering MI5 officers. And murdering them in a particularly gruesome and horrific way. A hammer is involved, as is a skinning-knife. The security services, however, are more concerned with the why than the how. Because it is beginning to look as if the killer is an insider, SAS-trained, one of the Regiment's own. The body count is mounting, and under strict cover of secrecy Alex is ordered to track down and eliminate the killer. To assist him in this task he is assigned an MI5 liaison officer, the attractive but abrasive Dawn Harding. And so begins a deadly and relentless manhunt.
The killer, Alex discovers, was almost certainly an undercover soldier codenamed the Watchman who, in the early '90s, infiltrated the highest levels of the IRA's Army Council. He was, and without doubt remains, a lethally skilful operator. But why is the Watchman slaughtering his way through the upper ranks of the security services? A nightmare chase, a betrayal, and a dawn firefight will all ensue before Alex learns the bitter truth: that in the shadowy battlegrounds of the Intelligence wars there is no good and no evil, there are only winners and losers.