John Smith, World Jumper Book One: Portal to Adventure
by E. Patrick Dorris
Waking, hospitalized with amnesia during World War I, "John Smith" quickly realizes he is not like other men. John can travel to parallel worlds, seemingly through the power of his own mind.
In this first serial, he not only learns of his power when it is triggered in the stress of combat, but visits an Earth still trapped in an ice age.