Jimgrim by Talbot Mundy

It was not Aububah. It was someone in the pink of condition, who could spare the breath to whistle softly to himself as he climbed in the oppressive heat radiated by the pyramid. It was someone who knew the way perfectly, to whom almost total darkness presented no obstacle whatever. And it was someone either utterly devoid of caution or else so sure of himself as to feel that caution would be out of place.

The moment his head reached about the level of our feet Grim flashed a light full in his face, but he took no notice of it. He was a white man, wearing dark, smoked spectacles. He had a short brown beard, carefully trimmed, and was very neat in his whole appearance. His hands were in the hip-pockets of a suit of tussore silk, well tailored. Except for the spectacles he looked like one of those athletes who refuse to grow old and retire; a man of means, perhaps, who delighted in mountain climbing, or perhaps an explorer. He looked like a man whose self-assurance was the result of the achievement.