This is book 9 of the "Modesty Blaise" series.

This note contains spoilers for the book.

Ferrying a sailboat single-handed, Modesty pulls a guy out of a dinghy. He's in bad shape, physically and somewhat mentally. But he appears to be Luke Fletcher, a very famous artist who dissappeared mysteriously a couple of years ago.

Modesty ends up involved, of course. An Australian millionaire with no taste is collecting great works of art, and occasionally collecting people from the art world to appreciate them (since he doesn't himself). He was trying out a brain-washing approach so he didn't have to kill off the artists, but it didn't quite work out.