Tarp is an agent with the CIA.
He was, at least, for far more years than he likes to think about, before he decided he had had enough of the baloney and the double dealing and the killing and stepped away. As the two-book series opens, he is living in an isolated house along the water border between the U.S. and Canada in the upper northeast and deeply enjoying the seclusion. He is not happy, so he claims, when his old life comes calling to him for more work but then he gives in fairly quickly so perhaps the peace and quiet was a bit too quiet.
In the opening of the first book, the history of Tarp seems a tad confusing as an old mentor and would-be employer, a former head of the British MI-5, comes to call and talks about trying to recruit him. But Tarp's bosses during his many years being an operative was the Agency. There is no doubt, though, that his friendship with this aged spymaster goes back quite a few decades.
When he was an operative, he worked all over the world but his biggest area of expertise was without a doubt China and his ability to act in that oriental nation is impressive considering he is Occidental. He understands the Chinese attitudes and methods and this knowledge kept him alive far more years than most in his field.
As the adventures progress, it also becomes clear that the number of enemies he has made is impressive as he was very good at his job but even many of those who would have happily killed him if the opportunity came respected Tarp for being so good for so long.