The story follows the character of Shadow, a man who has spent the last three years of his life in jail. On his last day behind bars, Shadow is told by the warden that his wife has passed away. Feeling lost and confused, when he is confronted by the strange Mr. Wednesday with a job offer, Shadow accepts (though it takes some persuading). However, Shadow has no idea that he is going to become involved in the lives of the gods. He and Mr. Wednesday travel around the US, trying to persuade the old gods (such as The Morrigan, Easter, etc.) to join their battle against the new gods (technology, gold, drugs, etc.).
I really enjoyed how the gods were depicted in the novel. The old gods were brought to the US, traveling in the minds and hearts of those the worshiped them. However, sooner or later, their beliefs in the old gods waned and the gods lost their power or disappeared altogether. These old gods get by on whatever the can: becoming con-men, resorting to stealing and prostitution, and things of that nature. America is again and again referred to as a land that "has no time for gods," and Gaiman does an amazing job personifying this idea for his readers. Although the old gods are battling the new gods, there is evidence that these new gods are beginning to lose their power and influence (such as robber barons). The old gods are battling against progress, but all of the gods are battling for some sort of belief or faith from the common man.
Gaiman lends a rather eerie tone to his writing. There is no shortage of ominous passages, gory scenes, or cryptic metaphors. There are some absolutely beautiful and haunting moments in the book, passages that I would go back and reread over and over. I've found that this is a common trait throughout all of his books, even in his YA novels such as Stardust. Here's an example from American Gods, that I think sums it up pretty well:
"The man he was following took his long stick, which Shadow realized now, as it moved, was actually a spear, and he slashed at the dog's stomach with it, in one knifelike cut downward. Steaming entrails tumbled onto the snow. 'I dedicate this death to Odin,' said the man, formally.'
'It is only a gesture,' he said, turning back to Shadow. 'But gestures mean everything. The death of one dog symbolizes the death of all dogs. Nine men they gave to me, but they stood for all the men, all the blood, all the power. It just wasn't enough. One day, the blood stopped flowing. Belief without blood only takes us so far. The blood must flow.'"
Apart from the writing style and the idea behind the book, I really enjoyed the characters Gaiman brought to life, and the little details that didn't seem to matter at first but would come back in a big way. Scenes that I thought were nothing but a break in the actual story came back in a surprising way in the end. By far, I think this is my favorite Gaiman book of the bunch.
I did read The Anansi Boys before I picked up American Gods, but I don't think it really ruined anything for me in the least bit. I don't think Anansi Boys is really meant to be a sequal, as there is a different tone to the story altogether. There is nothing at all about the fading powers of the gods, if I remember correctly. American Gods is much darker than Anansi Boys in that way.