Lately I've been hooked on Ben Bova's science-fiction books, from his RED MARS trilogy to the recent DEATH WAVE. ORION, published over thirty-years ago, is much more of a fantasy novel, as if he was channelling Philip Jose Farmer (TIERS series).
The main character is John O'Ryan, who we're introduced in the present day. He has some odd abilities, like being able to control his body's functions, like heart rate and breathing, and he has no memory prior to three years ago. This all changes when he encounters two very opposite men, who turn out to not be human, exactly. One is Ormadz, "the Lord of Light", a being responsible for Orion's predicament and the other is Ahriman, the brooding, bitter "Dark Lord". Orion is thrust back in time, at first to the time of the Mongols, then a primitive human settlement, and finally to the "war" that changed everything on this planet for us humans. What complicates things is there's a woman he encounters each trip, who seems to be reincarnated each time period and is directly connected to him, for some reason we eventually discover.
ORION works as both a fantasy story and a time travel tale. Highly recommended. I'm already reading the second book in the series…