For those who know better, or at least are willing to think about the subject, Paul Fussell's Class: A Guide Through The American Status System is a fascinating and often hilarious read. Fussell takes the standard three classes and expands them into nine, with a bohemian "classless class" tacked on at the end, and talks about why two families with the same income and living next door to one another, can be different classes. Since the book came out in 1983, a few details have become outdated as fashions changed, but in general the class indicators for clothing, homes, entertainment, food, and other things seem surprisingly stable. It's all the sort of thing that you may have never thought about, but when these characteristics are pointed out to you, they are immediately obvious.
Anyone who won't be crushed to find themselves showing up in a different class than they might have imagined would enjoy this book -- be sure to try out the "Living Room Scale" and see what your living room says about your status.