A Case of Conscience won the Hugo Award for Best Novel. And for good reason.
If I might generalize a bit, a lot of science fiction seems to be implicitly or explicitly hostile to religion. This book is one of the few, perhaps the only, science fiction book I've read that actually treats a current day religion with something approaching not just toleration, but outright admiration.
Our protagonist is Father Ramon Ruiz-Sanchez, a Jesuit priest and biologist assigned to the planet Lithia along with three others. Their mission? To make a recommendation back to Earth on whether or not contact with the planet should be allowed and if so, how much. Lithia is notable for having an intelligent, lizardlike species dominant there. One that has achieved fairly high technology - in some respects better than human technology - despite a lack of metals.
I'd say more, but that would only spoil this wonderful book. Blish packs a lot great stuff into this small package. The writing and pacing are excellent. The characters are strongly portrayed. Blish does an excellent job of throwing out tidbits about the societies that add flavor to the book without bogging it down. He never gives the impression that he is engaging in scientific speculation for its own sake. Good examples here are the description of how the Lithians had mastered the use of static electricity for power, and a party scene back on Earth that gives a good flavor for some of the dysfunction of human society. Oh, and did I mention that the actual story is great? Despite that fact that much of the book is conversation or introspection, a lot happens, and very little of it is predictable. A real page turner.
But best of all is Blish's character Ruiz-Sanchez. As I said, he is both a priest and a scientist. This would seem to set up a story based on the internal conflict between the two. Instead, Father Ruiz-Sanchez manages to embrace both simultaneously, believing that ultimately in a universe created by God, there is no rift between faith and science. This was a truly compassionate and sensitive portrait I thought.
Good writing, good characters, and a good plot. What more could you want in a book?.