Roy Blount wrote the main article, and it's worth reading from beginning to end. Blount's "good old boy, gone up North" schtick can wear thin sometimes, but there's not a hint of it in this piece.Stephen Carter wrote the inevitable piece on Twain and race. See "Getting Past Black and White".
They've got a piece on Twain's travel writing. Check out the map of his travels and lecture tours here.
And if you're interested, look at this collection of portraits.