The Practice Effect by David Brin

Physicist Dennis Nuel was the first human to probe the strange realms called anomaly worlds-altemate 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 struggled to solve the riddle of this baffling world.
David Brin, author of SUNDIVER and STARTIDE RISING, has been called "a major new talent" by Locus and "one of the brightest lights on the SF horizon" by Gregory Benford. In THE PRACTICE EFFECT, Brin combines all the rich characterization and state of-the-art science of his previous novels with new levels of fantastic imagination and rousing high adventure.