Neverwhere by Neil Gaiman

Simply put, my favorite book ever. EVER!
This book is the reason I focused on being a writer, the reason I'm not afraid to tell any story, the reason I keep reading.
To paraphrase Mike Meyers in Austin Powers 2:
Mr. Gaiman, I don't have those tendencies, but if I did you'd be on my list!
It's hard to describe what's so great about this story. It's nothing new, nothing that hasn't been told before in some shape or other...and maybe that's it. There's a primal quality to it, a fresh take on an ages old tale. Hercules, Orpheus, Gilgamesh, Arthur Pendragon, The Matrix...you've heard this story before...but never like this.
Richard Mayhew is an everyman, one of those heroes destined for greatness whether he likes it or not. He meets the Lady Door, a noble from London Below (a city that exists just, as the name suggests, below London, in the shadows and dusty corners...), and he is dragged into her quest to solve the mystery of her family's murder. There's not a character in the book that you won't love, most especially Mr. Croup & Mr. Vandemar, whose viciousness is matched only by their sheer likeability.
What's best about the book is the fresh take it will give you on the world you live in. You'll never be able to look at a manhole cover, alleyway, shadow, or darkened doorstep again without thinking...
Maybe. Maybe just on the other side there's something different. Something terrible, lurking, dangerous, and fantastic. And you'll find yourself hesitating, daring to take that step, test it, and find out.