Thank you, Wells Tower, for recommending Gringos in GQ back in 2011.  The hilariously convoluted plot is completely beside the point—the main attraction is Portis's Southern deadpan writing style. I like this summary of his work from the New York Times:

Mr. Portis evokes an eccentric, absurd world with a completely straight face. As a result, there are not a lot of laugh-out-loud moments or explosive set pieces here. Instead of shooting off fireworks the books shimmer with a continuous comic glow.

In addition to several other novels (including, of course, True Grit), he also has a book of shorter pieces called Escape Velocity, which includes a great essay about Nashville: