Saberhagen's Pilgrim series

 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.