Shifter

From the back cover of the Roc first edition: "He was Galen Sword by day... but who was he by night? Galen Sword was known as a rich New York City playboy. The public didn't know that his Porsche contained state-of-the-art equipment designed to track down an extraordinary creature, or that Galen was actually on a dangerous quest to find another world... his world. Shadowy memories of being sent away from his beloved parents had given Galen an obsession: to get back to a parallel universe where he had been heir to a powerful dynasty - but had been born unaccountably and unacceptably human. To find his way home now, he had to capture a beast that crossed over from that world into the city's dark streets looking for prey: a werewolf... alive, deadly, and very real. But trying to stop Galen was a race of brutal, magical beings, armed with a power that was terrifying and perhaps unbeatable..."