Reading exposes us to thoughts. Reading more exposes us to complementary or contrasting thoughts - and to our own thoughts. Here are a few more of the titles that I consider especially thought-provoking, when set off against each other.
Orwell's 1984 (Signet Classics) and Delany's Babel-17 (in Babel-17 / Empire Star) share more than the use of numbers in their titles. Among other things, they explore the effect of language on the people who use it, and against whom it is used.
If you like science fiction, try reading The Island Of Dr. Moreau then Sundiver (The Uplift Saga, Book 1), and the other books in Brin's "Uplift" series. Both address moral issues in granting human-like intelligence to other species - obviously, without their consent. Wells's story is an anti-vivisectionist horror, Brin's carries the hope that humankind might find (or make) different kinds of minds to talk to.
On a more serious note, compare The Illustrated Art of War to Macchiavelli's The Art Of War. Despite the similarity of name, they're very different books. One is practical, modern, and versatile - Sun Tzu's that is, roughly 1500 years older than Niccolo's.
Continuing the political theme, try Rand's Atlas Shrugged and Doc Smith's Subspace Explorers (Ace SF, H-102). Despite their different moods, both present bloodsucking leeches of the body politic who live off the creativity and intellectual wealth of others. The heroes, in both cases, are hardworking engineers and others, wielding democracy and hard currency as their sword and shield.
Utopian stories show people's aspirations at their highest, but sometimes also show rational thinking at its worst. See the difference between News from Nowhere and Other Writings (Penguin Classics) and Travels in Icaria (Utopianism and Communitarianism). They both take place in socialist ideal worlds, with similarities down to silly tangential romances. The latter, however, shows how totalitarian ends can come from the most benign of intents. While you're at it, take a look at A Modern Utopia. Wells distinguishes his utopia from just about all others, and especially Icaria, by dealing realistically with the faults of people as they are, rather than counting on some transformation of people into what they communistically ought to be.
This is getting too serious, though. Get your feet wet with Zero Girl and Warm Water Under a Red Bridge, two strange ones that include mysteriously damp shoes. OK, that second one isn't really a book, but it's my list and I get to pick.
Also in the silly vein, compare The Pro to Empowered, Vol. 1. Both take playful swipes at the superhero genre - the kind where a squeaky-clean savior carries out all kinds of heroic saves. Although both are female, the former isn't very squeaky, and the latter tends to stumble on the heroics. (She also has problems with her spandex suit and panty lines.)
If all the words are getting to you and you just want some pictures, compare Days of Intimacy to Layley. They seem so similar: black and white photo collections, by a single photographer and of a single model, and with a decidedly erotic (but not hard-core) sensibility. They couldn't be more different, due to Karsten's obvious affection for his model and her clear happiness in being seen.
Along the same lines, compare two collections by male photographers. Self-Images: 100 Women presents a hundred models as they want to be seen; 100 Naked Girls presents them as the photographer wants them seen. Despite the similarity in concept, Hegre's emphasis on youthful beauty and gentle eroticism contrasts sharply with the variety and individuality of Rival's models.
Illustrated fantasies can serve other purposes, too. Compare Mark Twain's The War Prayer to The Man Who Planted Trees. Both are short, profoundly moving, and illustrated in styles that carry the respective stories perfectly. The two moods, the Twain's darkness and Jiono's sense of peace, couldn't be more different, though.
You'll find philosophical explorations in science fiction also in Star Maker and The Making of the Representative for Planet 8 - in fact, Lessing is said to have been influenced by Stapledon. The similarities are loose at best, but both deal with devastation and transcendence. Lessing addresses it on a more personal scale, but Stapledon inquires into the biggest of all possible questions.
But enough frivolity - non-fiction demands attention, too. You might find analogies between the art history nerds of The Lost Painting: The Quest for a Caravaggio Masterpiece and the computer architecture nerds of The Soul Of A New Machine. Each group has its own arcane mysteries and thrills, barely comprehensible to most of us (well, SoaNM has meaning to me). Each author explores the lives of these rarefied minds, and makes the rivalries and rewards of those lives visible to the rest of us.
This is still a work in progress. I expect to add more over time, and I'm happy to hear suggestions for more interesting pairs. And, if you liked this, be sure to read my other suggestions in "Read books in pairs."