From Seven Seas:

The seminal novel from the writer of Vampire Hunter D!

For centuries, a secret peace treaty has existed between Earth and The Black World, a parallel dimension populated by shape-changers who possess awesome supernatural powers. Now that pact is up for renewal and a militant faction from the Black World will do everything in its power to stop the treaty from being signed. The fate of the world rests on the shoulders of a pair of special agents, one a human, the other a shape-changer from the dark dimension.