Split Infinity by Piers Anthony

Book One of the Apprentice Adept

In a world where technology and magic coexist, a master jockey named Stile finds himself caught between two realms: the scientific world of Proton and the magical land of Phaze. As he navigates these parallel universes, he discovers that his very existence is pivotal to the balance between them. With the help of a unicorn companion and his own unique abilities, Stile must confront powerful adversaries and unravel the mysteries of his dual identity to prevent a cataclysmic collision of worlds.