The Practice Effect — David Brin

Physicist Dennis Nuel was the first human to probe the strange realms called anomaly worlds -- alternate universes where the laws of science were unpredictably changed. But the world Dennis discovered seemed almost like our own -- with one perplexing difference. To his astonishment, he was hailed as a wizard and found himself fighting beside a beautiful woman with strange powers against a mysterious warlord as he struggles to solve the riddle of this baffling world.