In fact, it's not, and although I know that from reading James Atlas's biography and other sources, Bellow ended his life in favor of Augie's active spirit as opposed to the hopelessness and fatalism of the much shorter Seize the Day, it still seems that you don't have to live through the Great Depression or the current situation to recognize that by the numbers, the protagonist's life in Seize the Day much more accurately depicts life for many more Americans than the adventures of Augie. (Still, we could consider that's not the point at all, and that while both inform, it is Augie March that better entertains.)
And for the slim majority of Americans that do live their whole lives floating well above surface, housing and health benefits intact, etc--well, their adventures are not often with gigantic eagles, gamblers, thinkers, and Russian revolutionaries on the run, but rather, exist on planned vacations and shopping and HDTV where some other fellow is ramming his head into an incredible hulk of a defensive line.
As for the book itself, similar to Ravelstein, my interest waned when Bellow took his protagonist South of the border. Save for the Trotsky section in Mexico, I found most of the writing much more exciting when Augie was in America, Chicago for the most part, but not exclusively. I can't say why that it is, and it could surely be coincidence.
I can see why Bellow's friends and admirers were downright angry at the Atlas biography, and yet, frankly, it was good, easy reading, and in some ways, to me, the man's life was more interesting than his prose.
Another favorite nonfiction concerning Bellow is the chapter included in Brent Staples's Parallel Time, which although seemingly based upon a misreading or incomplete reading of Mr Sammler's Planet offers a great chase scene and is still fascinating in the way it brings perspective to the circumstances in which the canonical writer was living at University of Chicago in the late sixties and early seventies. I highly encourage a read of Staples's book concurrent with Sammler.
Bellow did come through with many great quotable sections of Augie, and I'll leave you with this one (for clarity, in short-quote format--with quotation marks, period in the wrong place, but alas, no identifying tag because I have no colon on this netbook and I just drove myself mad pasting in all the periods!--not because I'd want any of you to ignore MLA format for long quotations).
"Take the fact that people generally were full of loathing and it cost them an effort to look at one another. Mostly they wanted to be left alone. And they dug for unreality more than for treasure, unreality being their last great hope because then they could doubt that what they knew about themselves was true" (482).
And as we do, we move on, and now I'm reading number 197 of The Paris Review (great interviews with Samuel Delany and William Gibson, photography, and the next section of an early "lost" Bolano novel) and John Fante's Dreams from Bunker Hill.