I nominate
The Liquidator by
John Gardner.
I loved the 1966 movie based on this book (starring Rod Taylor, Jill St. John and Trevor Howard), so I read the book a couple of years later and enjoyed it very much as well.
There is a great deal of humor. A coward with little talent except for wooing the ladies is recruited to be Great Britain's primary political assassin. This results in his becoming a target of Britain's enemies.
This was the first of Gardner's Boysie Oakes series. Gardner then took over from Ian Fleming the James Bond books.
Kindle - Amazon US
http://www.amazon.com/Liquidator-Joh...dp/B00GGOBZXA/
Amazon UK
http://www.amazon.co.uk/Liquidator-J...dp/B00GGOBZXA/
Amazon CA
http://www.amazon.ca/Liquidator-John...dp/B00GGOBZXA/
Amazon Aus
http://www.amazon.com.au/Liquidator-...dp/B00GGOBZXA/