The Judas Goat — Robert B. Parker

Spenser had gone to London-and not to look at the Queen. He'd gone to track down a bunch of bombers who'd blown his client's wife and kids away. His job was to catch them. Or kill them. His client wasn't choosy.

But there were nine killers to one Spense -- long odds that could add up to a short life. Hawk, the iron-fisted Boston enforcer, would help balance the equation. The rest would depend on a wild plan. Spenser would get one of the terrorists to play Judas Goat-to lead him to the others. Trouble was, he hadn't counted on her being very blond, very beautiful, and very, very dangerous.