Outside the Oval Office is a small rectangular room with two side-by-side, nondescript wooden desks. In one sits President Obama’s personal secretary. In the other is Brian Mosteller, the man who sweats the small stuff so that the president doesn’t have to. Few have even heard of Mosteller, but if you look closely at photographs taken inside the White House, you can often glimpse him at the edge of the frame, omnipresent. From his chair, he is the only person in the White House with a direct view of the president at his desk. No one gets in the Oval...