I recently finished the thematic trilogy "After Such Knowledge" by James Blish.
The book consists of four novels: "Dr. Mirabilis", "Black Easter", "The Day After Judgement", "A Case of Conscience". The last of these was recently reprinted as number 30 in the Gollancz SF Masterworks series, which is kind of like Pokemon for science fiction nerds who don't also collect Pokemon. I just checked, I still have 28 out of 70 to read.
A while ago, I reviewed "Cities in Flight" (number 3 in the SF Masterworks series). I enjoyed the scope of those works (which spanned from the present day to the end of time) and Blish's approach (applying the lessons of history to the future).
I wasn't sure what to think when I started "Dr. Mirabilis". I was expecting science fiction, and instead faced lovingly detailed historical fiction, a biography of the 12th century friar and scientist Roger Bacon. Blish presents Bacon as a man obsessed with expanding scientific knowledge through experimentation, making him one of the first modern scientists. It is clear that Blish feels Bacon was held back in his accomplishments by the time in which he lived, and that his accomplishments are all the more amazing as a result. The drama of "Dr. Mirabilis" hinges as often on points of principle and ethical choices as well as points of science. It's challenging fare, and reminded me of the excellent "Canticle for Leibowitz" by Walter M. Miller.
By comparison, the rest of the series was much shorter and more straightforward. "Black Easter" and "The Day After Judgement" are two parts of the same story. They present a world in which magic is made possible by collusion with demons and angels. Dark magic is soul-damning but effective, and it is used to terrifying effect in both novels. I won't say anything about the specifics, other than to say that the ending of "The Day After Judgement" is both surprising and great.
The final novel ("A Case of Conscience") is a morality play set in space, and more than any other book I've read outlines how belief a deeply spiritual man's belief in God would be changed by an encounter with a world that defies church canon about the nature of life, God, and the Devil. It's understandable that this would be the only one of the series to be included in a list of science fiction masterworks, as it's the only one that includes the usual trappings of science fiction (rockets, alien races, and the simple fact that it's set in the future.)
Of the three, I would have to say that "A Case of Conscience" was my favorite. "Black Easter" and "The Day After Judgement" are short, and are worth reading in order to understand their truly great ending. "Dr. Mirabilis" is a long read and probably is more of interest to history buffs than science fiction readers. Still, it's a good book, and I'm glad to have read it.