"A Canticle for Leibowitz" by Walter Miller, Jr. I would be completely contented with a three- or six-part series, but I could also see it being turned (well)

"A Canticle for Leibowitz" by Walter Miller, Jr. I would be completely contented with a three- or six-part series, but I could also see it being turned (well) into a longer thing by interpolating. Similarly, but probably more accessible, "The Sparrow" by Mary Doria Russell (and it's sequel "Children of God"). It would definitely do better as a long series.