I'm pretty sure C.S. Lewis would have objected to calling his 1938 novel, Out of the Silent Planet, a "pulp fantasy" and perhaps rightly so. Indeed, his "Space Trilogy," of which this is the first volume, might in some ways be seen as a rejection of many staples of the sword-and-planet genre of which it might superficially seems to have much in common. It is, after, the story of an Earth man -- Dr. Elwin Ransom -- who journeys to another world and finds his unique talents essential to preventing a great evil from transpiring.
Of course, Dr. Ransom's unique talents are not superior strength, hardiness, or military training but rather his facility with languages. Ransom, whom Lewis may have based, at least in part, on his friend J.R.R. Tolkien, is a philologist who uses his linguistic knowledge to learn the tongue of the inhabitants of the planet Malacandra, as Mars is called by them. This enables him to learn from them the history of not only Mars but also of Earth, the "Silent Planet" mentioned in the book's title, so called because, until the events of the book, what transpires on it is a mystery to the other beings of our solar system.
Unlike, say, John Carter or Esau Cairn, Ransom's journey to Mars was as the victim of a kidnapping. An unscrupulous physicist by the name of Professor Edward Weston plans to travel to Mars both to mine gold (which the planet has in abundance) and to lay the groundwork for the eventual colonization of the planet by human beings. Of course, doing so will result in the subjugation and perhaps extermination of the native Malacandrans, a fate Ransom cannot countenance and against which he struggles in the course of the novel. Fortunately for him, the philologist has allies in unexpected places and they too reveal much about the true nature of the cosmos.
Out of the Silent Planet is what might be called "theological science fiction," for its main purpose is not describe an adventure story set on another world as to explore the theological implications of life on other worlds from a Christian perspective. Even if one does not share Lewis's religious convictions, it's nevertheless a fascinating book purely as a bit of speculation about humanity's relationship to and responsibilities toward other intelligent life forms. Likewise, the cosmology Lewis outlines, which draws heavily on medieval notions of the universe, is very well done and, as others have noted already, was an inspiration to me as I worked on my planes-as-planets notion. I suspect others will be similarly inspired.
I have a great fondness for the Space Trilogy, which I first read in high school. I was never a fan of the Narnia books, which I found unsubtle and frankly boring. The Space Trilogy, though, was much more to my liking and, while not without its flaws, both as bits of speculation and as novels, their virtues far outweigh them. The middle book in the trilogy, Perelandra, set on Venus, is in my opinion the least interesting of the three and the one where Lewis's weaknesses as a writer become most apparent, but Out of the Silent Planet and That Hideous Strength, the final book in the series, are among my favorite novels. They're both well worth reading if you've never done so.