Knock Down by Dick Francis
It's the classic law of the invader. Get to the strongest guy and smash him, and then watch the weaker ones fall like ninepins. Which is exactly what a vicious gang of horse dealers decides to do when their little kickback scam is being threatened with a spot of old-fashioned honesty.
But they haven't reckoned with bloodstock agent Jonah Dereham, a man who belies his quiet exterior with a steely and implacable resolve. They can threaten him, try to burn down his home, even attempt to kill him. But they'll soon wish they hadn't.
Because Dereham flushes out the real scam, and with it finds out just who the real Mr Big is. And when he does that, life suddenly becomes cheap...
'Another wonderfully effective thriller' Sunday Times