Lord of Rays by Robert F. Young

Lt. Comdr. Guest is alone aboard the spaceship Helios 5, orbiting close to the sun, when he spots a strange ancient Egyptian funeral boat. On the boat lies a mummified figure wearing the mask of Osiris. As Guest watches, a man appears reading from a scroll, then vanishes. Guest starts acting strangely, speaking ancient Egyptian words.

Moonbase contacts Guest, warning him that he's entered an intersection of realities—the physical world and the ancient Egyptian underworld. With time running out before his scheduled return, can Guest escape the grip of the past?