The back cover of my copy of A Canticle for Leibowitz calls it "a novel that transcends genre." But if you wanna get technical, the 1961 Hugo winner is almost the antithesis of science fiction.
As we discussed a month ago, there's a decades-old argument over the role of science in science fiction; the corollary debate is whether the genre's name should be changed to something like "speculative fiction" or "philosophical fiction," since so many of the stories that comprise it explore alternate realities, possibilities, and ideas ably and deeply while barely touching on hard science at all. There are problems with those other names too, of course — most notably, what wouldn't qualify under them, since what kind of fiction doesn't explore alternate realities? — but A Canticle for Leibowitz could serve as evidence that the name needs to be changed somehow. Because in the novel, science isn't responsible for anything really happening.