It would either be this, since it appears to be a standalone, or what used to be called Rain Fall (he retitled all his books, so it's now called A Clean Kill in Tokyo), which is the first of the rest of his books, which are all in the same series. He had a standalone series as well, but it ended up merging with his main series.
The Rain series is about an assassin who specializes in "natural causes" deaths. Eisler himself is actually former CIA, and he brings his knowledge into his writing (as far as a civilian like me can tell). It also takes him forever to write his books because he likes to scout out locations and take tons of pictures of them, so he can describe them in intricate detail.