back cover of the 2007 Penguin Classics edition of A Princess of Mars by Edgar Rice Burroughs has a prominent blurb by Carl Sagan: "Might it really be possible—in fact and not fancy—to venture with John Carter to the Kingdom of Helium on the planet Mars?" Although the cover does not specify the source of the quote, it's from the "Blues for a Red Planet" chapter from Cosmos; references to his being a fan of the John Carter books since first reading them as a kid appear scattered throughout Sagan's writings, including an anecdote about obtaining a related vanity plate (due to a limit of 6 letters per plate, he had to settle for "PHOBOS" instead of his first choice, "BARSOOM"). And this hasn't been the first time that Burroughsians have noticed Sagan; for instance, consider the Burroughs fansite ERBzine's lengthy tribute to Sagan.
I'd be happy to see more Sagan blurbs on other science fiction books; he had a generally favorable view of science fiction in general (with some caveats about scientific errors and the promotion of pseudoscience) and had nice things to say about quite a few science fiction classics in his writings, for instance in the essay "Science Fiction—A Personal View" in Broca's Brain, or the extended discussion in Pale Blue Dot (which I was quite surprised to see when first reading it) of a 1942 short story by Jack Williamson dealing with antimatter and asteroid colonization, "Collision Orbit" (which was incorporated into the fix-up novel Seetee Ship). (By the way, I've always wondered if the character Jane Carter in Williamson's The Humanoids was a Burroughs reference, with the character's teleportation abilities being a takeoff on Burroughs's use of astral projection to get John Carter to Mars.)