First in a series by Rob Thurman... I've seen it labeled both as the Cal Leandros series and the Nightlife series. It falls into that contemporary/urban fantasy genre.
So yeah, one of the characters is half human, half Auphe/Grendel/monster, and there's werewolves and a vampire and Robin Goodfellow (ya know, from A Midsummer Night's Dream?) but mostly I think it's about these two brothers and how much they love each other. They really only have each other and they're both desperate not to lose the other. It's a really sweet relationship.
Basic plot stuff so I remember the book... the Auphe want to go back to a time before there were so many humans, but they can't do that on their own and of course their plans involve our protagonists.