Pilgrim Series by Fred Saberhagen

When college student Tom Scheffler agrees to care for the luxury Chicago apartment of his great-uncle Montgomery Chapel, he soon realizes that traveler Chapel’s apartment contains more than a priceless collection of exquisite Egyptian artifacts. An alternate Egyptian world awaits Tom, a world of ancient gods, uncountable treasure, and danger. A world where revenge will be had. A world very much of interest to a dangerous fugitive and time-traveler named Pilgrim.

Who is Pilgrim? Tom Scheffler and Jerry Flint are going to find out the hard way, because the mysterious Pilgrim is going to have them fighting for their lives centuries, millennia before they were born. Pilgrim is the rogue time traveler trying to change the course of history for his own ends. And even if the two hapless adventurers survive Pilgrim's deadly game, they have no assurance that they will ever see their own timeline again.

Pilgrim is a Lost Traveller, adrift in time and reality. His only hope for returning home lies in subtly altering the history of our own timeline to more closely reflect his own. His current project: to prevent the assassination of Abraham Lincoln--After the Fact!