The Great and Secret Show by Clive Barker
Down-and-outer Randolph Jaffe works in the dead-letter office in Omaha. Reading through the mass of mail, he finds clues to an alternative reality, the laws of which are called "the Art." Mastering these principles, he becomes powerful but evil, and presses into service a man named Fletcher, who synthesizes a transforming drug, the Nuncio. Later understanding the corrupting nature of his creation, Fletcher rebels against Jaffe, and the two, now demigods, engage in a cosmic struggle. To enlist allies, each sires offspring (using the seed of mortal men), and their spiritual children help to carry on the bizarre battle.
First part of a trilogy, but it is a complete story that stands entirely on its own. Probably a little pricey, though...