The Practice Effect by 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.