Lord of Rays — 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?