Peter O'Donnell
Peter O'Donnell also wrote as Madeleine Brent.
http://www.cs.umu.se/~kenth/modesty.html
is an excellent resource on this author.
To help keep the novels and the adventure strip collections separate, here's some info about the Modesty Blaise works.
In 1963, O'Donnell began his 38-year run as writer of the Modesty Blaise adventure story strip, which appeared six days a week in English and Scottish newspapers. He retired the strip in 2001.
Meanwhile, during those 38 years, O'Donnell also wrote 13 books about Modesty Blaise: 11 novels and 2 short story/novella collections. These stories are not related to the strip stories; they are not novelizations of strip stories. They are entirely new, though the characters and "lives" are the same. These have been labeled "series #0".
There is a large article on Peter O'Donnell on Wikipedia, with a complete bibliography.
“On the whole I try to keep Modesty and Willie in timeless settings, which is why I avoid all the latest slang and in-words. It won't be long before 'brill' sounds as dated as 'super' does now. [Uncle Happy, 1990]”