Time Travelers Never Die — Jack McDevitt

When physicist Michael Shelborne mysteriously vanishes, his son Shel discovers that he had constructed a time travel device. Fearing his father may be stranded in time-or worse-Shel enlists the aid of Dave MacElroy, a linguist, to accompany him on the rescue mission.

Their journey through history takes them from the enlightenment of Renaissance Italy through the American Wild West to the civil-rights upheavals of the 20th century. Along the way, they encounter a diverse cast of historical greats, sometimes in unexpected situations. Yet the elder Shelborne remains elusive.

And then Shel violates his agreement with Dave not to visit the future. There he makes a devastating discovery that sends him fleeing back through the ages, and changes his life forever.