American Gods
I can't do a review of this justice because I read it 6 or 7 years ago, but the basic premise is that various gods and goddesses are trying to survive in a modern world that no longer reveres or even acknowledges them. Very interesting take on petty immortals wearily resigned to a pseudo-mortal existence.
Oh, this book. Here, we have an ordinary man finding himself on an extraordinary adventure. Sounds generic right? This book isn't, I promise. London is separated into 2 sections: the one everyone sees, London Above, and the one that exists overlaid in impossible space (think the Tardis) under the city: London Below. It's where everyone and everything that has slipped through the cracks lives. If you're interested, a radio drama of this book exists and may feature a certain Benedict Cumberbatch, James McAvoy, and Natalie Dormer as main characters.
I don't think I can recommend this book enough. It may not be possible. This book is about a prissy angel who loves to read and a lackadaisical demon, both of who don't really want the Apocalypse to happen. All of the characters are well rounded and memorable, with witty lines aplenty. I think one of my favorite lines of any book ever came from this one, when describing how Crowley became a demon: "He didn't fall so much as saunter vaguely downwards."