So, what is this book about? New Gods versus Old Gods, yeah?
A god can only exist if that god is believed in. As belief wanes, a god's power is diminished. If a god is forgotten, that god is dead and gone as if it never existed. For some reason, Gaiman's gods have physical bodies, and can be physically killed, which seems a little (VERY) inconsistent with the whole idea of this book, but let's put that aside for now.
So the old gods (Odin, Loki, Kali, etc.) are languishing in an unbelieving modern America. New gods are rising (The Internet, TV, Media, Celebrity, Drugs) and seeking to wipe out the old.
- By the way, here is a great guide to each character and the god they represent: http://www.frowl.org/gods/gods.html
It's interesting to note that Gaiman is English and the book seems very cautionary and subtly anti-american. The old gods (mostly Norse, European, and Eastern) are shown as the wise old gods of tradition. The new American Gods are shown as immature, young, cocky, violent things. Wednesday (Odin) says "this country needs it's legends" meaning "this country is worshiping the wrong gods and forgetting about the right ones."
I'm halfway through the book. It's an interesting idea and obviously Gaiman did his homework on a huge number of myths, gods, dwarves, elves, leprechauns, and other spirits. I just don't know if he can pull it all together. It feels like he is playing WAY too lightly with big heavy ideas. I want to say he's completely out of his depth here but maybe the rest of the book will prove me wrong.