The Baron and the Unfinished Portrait — John Creasey

As John Mannering's wife paints a portrait of a celebrated beauty, Lady Deirdre Vandemeyer, she begins to doubt the authenticity of her subject. Then she is stabbed and Mannering (aka 'The Baron') goes undercover as a personal assistant to try and get to the bottom of the mystery. What he discovers sets the mind reeling and as always keeps the reader enthralled to the very last moment.