The Christmas Party by Rex Stout:

Opening lines:

I decided to count ten, and swiveled my chair, facing my desk, so as to have privacy for it. As usual when we have no important case going, we had been getting on each others' nerves for a week, and I admit I was a little touchy, but his taking it for granted like that was a little too much. When I had finished the count I turned my head, to where he was perched on his throne behind his desk, and darned if he hadn't gone back to his book, making it plain that he regarded it as settled. That was much too much.