The Galton Case, by Ross Macdonald (1959)

She had...a look on her face you don't see too often any more, the look of a woman who hasn't been disappointed...

Lew Archer is hired by a wealthy-yet-dying matriarch (through a lawyer friend) to find an estranged son who has been missing for nearly twenty years. The lawyer confides to Archer that there probably isn't any hope in succeeding and that he (Archer) shouldn't really break his back trying. But within twenty minutes of accepting the case, the lawyer's smart-alecky manservant - a man by the name of Culligan - is killed by an intruder. And the prime suspect makes a getaway in Archer's car.

Not believing in coincidences, Archer ploughs on.

before the book's halfway point, the moldy bones of the missing son are found, and so is his alleged son - the matriarch's hitherto unrevealed grandson. And further complications ensue. In trying to determine in whether he's legitimately the heir, Archer is savagely beaten up and spends a week in Reno sipping his meals through a straw.

Misleading photo cover of this edition aside, Archer rarely gets mad. And he rarely takes things too personally. He does both when he comes across one of his tormentors in Canada (!) a few chapters later.

     Without giving the matter any advance thought, I set myself on my heels and hit him with all my force on the point of the jaw. He went down and stayed. His brother knelt beside him, making small shocked noises which resolved themselves into words:
     "You had no right to hit him. He wanted to talk to you."
     "I heard him."
     "He's been drinking, and he was scared. He was just putting on a big bluff."
     "Put away the violin. It doesn't go with a knifing rap."
     "Tommy never knifed anybody."
     "That's right, he was framed. Culligan framed him by falling down and stabbing himself. Tommy was just an innocent bystander."
     "I don't claim he was innocent. Schwartz sent him there to throw his weight around. But nobody figured he was going to run into Culligan, let alone Culligan with a knife and gun. he got shot taking the gun away from Culligan. Then he knocked Culligan out, and that's the whole thing as far as Tommy's concerned."
     "At which point the Apaches came out of the hills."

One of the better Lew Archers. Of course, I really haven't come across a dud Macdonald yet...