Thanks to last week's commenters, I think I've got a reasonable approach to the works of John Barth.
Sot-Weed was highly praised although one called it "annoying."
There was also some regard for Barth's early novels The End of the Road and The Floating Opera, although also cautions that the latter was atypical of his work (less playful, more existential in tone).
Barth's later novel Chimera was also recommended, and I was instructed not to read LETTERS until I had tackled the preceding books. As one who regularly instructs readers not to read The Savage Detectives until they've read the short novels available in English, I take this to heart.
So, it looks like I will do this in order. Since the omnibus version of The End of the Road and The Floating Opera is scarcely half the size of Sot-Weed or Giles, it shouldn't be too much trouble to read those first. Then I'll take my chances with Sot-Weed, and, depending on how I feel, either jump straight to Chimera or stop off at Giles first, before concluding with LETTERS.
And now for fun I'll quote Gore Vidal's estimation of Barth, from his noted and lengthy essay on virtually every important postmodern American novel published at the time:
Barth was born and grew up a traditional cracker-barrelly sort of American writer, very much in the mainstream—a stream by no means polluted or at an end. But he chose not to continue in the vein that was most natural to him. Obviously he has read a good deal about Novel Theory. He has the standard American passion not only to be original but to be great, and this means creating one of Richard Poirier's "worlds elsewhere": an alternative imaginative structure to the mess that we have made of our portion of the Western hemisphere. Aware of French theories about literature (but ignorant of the culture that has produced those theories), superficially acquainted with Greek myth, deeply involved in the academic life of the American university, Barth is exactly the sort of writer our departments of English were bound, sooner or later, to produce. Since he is a writer with no great gift for language either demotic or mandarin, Barth's narratives tend to lack energy; and the currently fashionable technique of stopping to take a look at the story as it is being told simply draws attention to the meagerness of what is there.
To help you calibrate those rather harsh sentiments, this is the same essay in which Vidal dismissed Gravity's Rainbow (after highly priasing V) as "Removed from the academic mainstream and its extra-terrestrial connections, Pynchon's Gravity's Rainbow is just an apocalyptic 'whine.'"