Honestly, I've never read anything like this before.
American Gods is sweeping, complex, and fast-paced, with side-stories and back-stories sprouting from every chapter, as if wishing to become novels in their own right. I can see why this book won twenty thousand awards and why everyone has recommended it.
In a sense, Gaiman here does for America in fiction what Antonín Dvořák did for it in music at the end of the 19th century: highlight and artistically adapt the country's cultural artifacts, which were in plain sight all along. Gaiman imagines the dislocation that comes from immigrants abandoning their beliefs in order to assimilate, and what happens to those "gods" when they are left behind. It's the idea that nothing, not even intangible beliefs or ideas, ever really vanishes.
Set against this grand backdrop of gods and battles for American hearts and minds is the story of one man who must navigate both the tangible and intangible worlds as he tries to understand what exactly the gods want with him. Oh, and his dead wife kind of comes back to life (but she's still dead) and he keeps having these extremely bizarre vision-dreams...
Gaiman invites us to broaden our understanding of history and accept that America might have been visited by various peoples far earlier than previously thought. In my opinion it is this, plus Gaiman's ability to bring this shadow-world to life, that makes American Gods so successful.