Not Exactly Dea‪d‬

Richard Hugh, aka Triple-O Five of the unsung British agency nicknamed MI5 1/2, is happily making life miserable for bad guys and, when he has time, cultivating a reputation as a lady's man. But then the Americans get the notion that he's been marked for death by a notorious spy killer who has been bumping off spies belonging to an odd mix of nations. This would almost be business as usual, except that the Americans send Emma Chapman over to keep an eye on Hugh, in the hopes of catching the assassin (preferably before he bumps off 0005).

Chapman is short, plain, and respectable, and couldn't care less that Hugh is suave and handsome. This is not at all the sort of female he's used to working with.
Before long, the Hugh-as-rat-bait hunt for Mighty Planetary Master also involves the French, represented most notably by the fearless and philosophical Leandre Durand, who doesn't mind in the least keeping his own counsel.

This is the first book in the MI5 1/2 series. The second book is Not Exactly Innocent.