The Accidental Ambassador by D A Barr

From the back cover: "The scout had performed this task many times, and it was good at its job. The original plan was simple. It was carried out after several years of studying the subject and careful consideration of the variables involved. It failed. Plan B was made on the fly . . ." The scout was not expected; it's colleagues had not informed the subject, Douglas, of the scout's arrival. In fact, the colleagues had vanished. Cold calls like this seldom went well, but the circumstances required that the attempt be made. Douglas introduces someone to the scout; someone unexpected. Just to make this mission more complicated, someone else on Earth wants the scout dead.