The Corridors of Time by Poul Anderson

The corridors of time connect the ages to each other. Through them, one can travel backwards and forwards over the history of man. But rival factions have waged war for centuries: the gates onto time are bitterly fought for and jealously guarded. Malcolm Lockridge is a modern American swept into the past by the beautiful and mysterious Storm Darroway, the lithe, glowing Goddess of her people. Malcolm is an ordinary man of today, caught up in a time war beyond his comprehension.