Interesting in a thousand ways. To pick, somewhat at random:
1. Guy Davenport dwindles at the end, withering into crotchety invisibility as Kenner goes on, if anything gaining in brio. The last 30 pages or so are thus weirdly like listening to half of a telephone conversation - and it's not the half that we prefer.
2. Why is Kenner, who would seem by far the alpha of this relationship, so obviously the poorer writer? This question has bothered me for years. There's just something subtly wound up about him - a pitch to his prose that makes it seem to be constantly aiming ever so slightly past the reader (not edificatingly beyond, either - he thinks he's being direct, but he's constantly losing us). Meanwhile, by comparison G.D. is one of the most colloquially-suggestive writers ever. His manners are superb; he knows just when, and how, to navigate our attention. Above all, he seems to constantly have us in mind. Kenner is always just 10 degrees more involuted, and that 10 degrees makes a big difference.
3. That said, I came to (begrudgingly) really like H.K. by the end of the books. He's a kook, with his crazy hair and absolute devotion to the Cantos. Yes, there is the politically conservative, W.F. Buckley Jr.-bro-ing streak, but it comes off so frequently as clumsy. You never get the sense that his conservatism is actually "conservatism." He's taking one step back to take two steps forward.
4. It is literally absurd how much these two accomplished and knew. At the same time, they were both charlatans. I mean that in the best possible way, and close readers of these letters will know what I mean: they both admit as much to one another over and over again. For all that the two of them denounced pseudo-scholarship and fakes when they encountered them, they themselves were largely building castles out of air.
5. In some ways, their art and scholarship both profit mightily from Pound's old idea (in his letters somewhere?) that you can gain a lot from just choosing some random minor or under-read writer and reading everything he/she wrote. Kenner's list of totems is flashier, partly because his famous efforts raised the prestige of everybody. But Davenport is the master Reader of Random Oeuvres. Doughty, Santayana, Wilkie Collins, O'Henry - the list goes on and on. And he is no mere collector - he doesn't just possess these works as curios for his shelf: he thinks and feels through them, absorbs them, incorporates them into his larger sphere. This to me is the most appealing aspect of their Renaissance-manliness.
6. Their "other interests": both men had passions that were extra-literary, but which they used to enhance and shape their views on literature. For Davenport, this was most obviously drawing, painting, film. For Kenner, drawing again, cartoons, technology, and the weird Buckminster Fuller stuff.
7. Buckminster Fuller
8. The Stoic Comedians, an early collaboration, is a key to both men's work, in that it looks both forward, into Joyce and Beckett and all the high modernist stuff we recognize, and backwards, to Flaubert and the 19th century. The comedy of logic was a key theme to both H.K. and G.D., and their performance of the mind depended on it. Also: Kenner is never funny, although frequently witty. Davenport is frequently funny and rarely stoops to wit.
9. Blind spots: women. This is not latecoming, historical-hindsight-type bias. Both writers seem to have had a legitimate problem accessing and benefitting from the alternative, more female-modulated traditions that were a big part of the story of their times. For example, neither can see Virginia Woolf. Like, shockingly, they seem to be literally unable to see her. Exceptions: Marianne Moore, Eudora Welty. Also, for what it's worth, when circumstance brings unexpected art into their kens, the men always seem to be able to respond to it sincerely and without prejudice.
10. Both men were anti-transcendentalists, or at least on that side. The Melvilles and Thoreaus. The luminous particular.
11. The life of the mind is worth living. It is not boring or a waste. Casaubon was supremely happy working on his Key to All Mythologies. Interest is its own reward. At the same time, we all are alone. Good luck.