Dark Duet by Peter Cheyney (Pan, 1958)

(For the Fontana edition of the same book, go here.)

COUNTER ESPIONAGE!

IN THIS BUSINESS YOU'RE EITHER A BRAVE MAN ... OR A DEAD ONE

A look of intense surprise came over Mrs Marques's face.  Then her mouth opened.

Her face twisted in supreme agony for a split second; then she slumped sideways on the settee.

You have to be as tough as seven devils in hell for Process 5 ... but it's artistic.

"Process 5" is murder... of course!