John Kendrick Bangs 
R. Holmes & Co. [PDF ebook] 
Being the Remarkable Adventure of Raffles Holmes, Esq., Detective and Amateur Cracksman by Birth

What would happen if master detective Sherlock Holmes sired a son with the daughter of one of his archenemies? That’s the supposition at the center of R. Holmes & Co., which pits criminal mastermind Raffles against Holmes and goes on to tell the story of how Holmes fell for Raffles’ daughter Marjorie, with whom he later had a child, the Raffles Holmes of the book’s title.