The Mask of the Sun — Fred Saberhagen

When Mike Gabrieli's neer'do'well brother Tom disappears shortly after discovering a fabulously valuable Aztec relic, Mike rightly suspects that this time the family's black sheep has got himself into the kind of trouble from which even Mike won't be able to extricate him. But still, Tom is -- or was -- his brother, and Mike must do what he can.

For Mike this is the beginning of an adventure beyond imagining, an adventure that will put him in constant peril of his life as he shuttles. between past, present and future of an alternate reality, fighting beside the descendants of the Incas as they battle to erase Pizarro's bloody footprints from the New World, and secure the reality of their own existence.

But is the "alternate reality" really an alternate, or is Mike actually struggling to erase the very future that gave him birth? The answer lies in the source of all his troubles and his only hope of survival: THE MASK OF THE SUN