Ghosts / Mark Dawson.

"John Milton is a ghost. He was Number One. The most dangerous assassin in Group Fifteen, the black-ops organisation that solves problems when diplomacy has failed. Now Milton is Her Majesty's most wanted fugitive--an anonymous loner with a deadly set of skills. So when he is arrested following a brawl in a Texas bar, the last person he expected to bail him out was a glamorous operative from the Russian Secret Service. Milton is blackmailed into finding his predecessor as Number One. But she's a ghost, too, and just as dangerous as him. He finds himself in deep trouble, playing the Russians against the British in a desperate attempt to save the life of his oldest friend. Some ghosts are better left alone. But these two have been disturbed and now there's going to be hell to pay. This, the fifth book in the best-selling John Milton series, is his most action-packed adventure yet. Read about the damaged hero that reviewers are describing as 'the English Jack Reacher'"--Amazon.com.