Carl Sagan wrote a great book, and movie, called Contact. I don't need to explain what else hes done, do I? I hope not.

A Connecticut Yankee in King Arthur’s Court by Mark Twain

Since you mention Discworld: Jack Cohen ^[1] (best known as a biologist) and Ian Stewart ^[2] (best known as a mathematician and popular math writer) co-wrote The Science of Discworld ^[3], then went on to write two sf novels, Wheelers ^[4] and Heaven ^[5], exploring alien biology.

Winston Churchill ^[6] wrote a short alternate history If Lee Had Not Won the Battle of Gettysburg ^[7]¹.

Jules Verne ^[8] is primarily known in France as a writer of adventure stories (Around the World in Eighty Days, Michel Strogoff, Mysterious Island, …) and technothrillers (The Survivors of the Chancellor, The Begum's Millions, …), but he wrote a few genuine science fiction tales (Robur the Conqueror, An American Journalist's Day in 2889 ^[9]).

¹ _{Yes, it is alternate history, in spite of the title.}

The Machine Stops ^[1] by E.M. Forster

Its a short story, not a book, though.

I haven't read the Space Trilogy by C.S. Lewis, but I'd say that's about as good an answer as I can come up with right now.

While I haven't read the first book Out of the Silent Planet, I know someone who did and I have heard the Iron Maiden song ^[1], so it can't be all bad.

Steven King wrote The Running Man ^[1] under the pseudonym Richard Bachman. It's a great story that bears very little resemblance to the Arnie movie.

(Don't get me wrong, the film was fun but the book is well worth a read too)

Brave New world ^[1] by Aldus Huxley. Even though this is his most famous work, he mostly wrote general fiction and poetry.

Nevil Shute ^[1] - On The Beach ^[2] a post apocalyptic tale set in Australia after World War III by an author more famous for stories such as A Town Like Alice.

Making History - Stephen Fry

What about a author Doris Lessing ^[1] who won the Nobel Prize in Literature and wrote the excellent Canopus in Argos ^[2] science fiction series.

Armor by John Steakley. His only other work was Vampires, which was made into a really crappy movie.

Children of Men by P.D. James.

Iain Banks' Transition

I think this counts because its published under his 'black and white' Iain Banks name, and not his colour/sci-fi name Iain M. Banks!

definatly SCI FI! Great book too.!

Someone has mentioned the Space Trilogy by C.S. Lewis already, specifically mentioning Out of the Silent Planet. I write another answer, instead of just a comment, to expand a little, but also highlight particularly a different book in that series.

Out of the Silent Planet, the first book, is old-school sci-fi, a bit of an homage to H.G. Wells: you will explore new worlds, imagine alien societies, things like that. Perelandra is similar but waxes much more religious-allegory. (If you're allergic to thinking about Christianity, you can very much survive OotSP, but Perelandra is definitely not for you. Skip it if you want; it won't particularly hurt. The recaps in the third one are limited, but it's kind of tangential to the main story.)

That Hideous Strength is the complex and fascinating one, though! Besides new main characters (and some of the old ones in the background), the setting shifts to Earth, post-World War II, and explores some themes common to that era. It will remind you of Farenheit 451 and 1984 -- it actually predates both of these (and the atomic bomb, to boot, if I'm not mistaken). You will witness the rise of a technocratic authoritarian dystopia. While the the modern world and scientific establishment has (quite fortunately) embarked upon a rather different direction since then, with luck the story can still retain a little bit of that old thrill:

There is nothing outrageously improbable in such a conspiracy. Indeed, at a moment when a single atomic bomb – of a type already pronounced “obsolete” – has just blown probably three hundred thousand people to fragments, it sounds all too topical. Plenty of people in our age do entertain the monstrous dreams of power that Mr. Lewis attributes to his characters, and we are within sight of the time when such dreams will be realisable.

-- THE SCIENTISTS TAKE OVER: George Orwell’s review of C. S. Lewis, That Hideous Strength (1945)

It's still C.S. Lewis, though. Orwell couldn't relate to the "supernatural" angle, and thought it did the book a disservice, but was otherwise quite satisfied. You can't please everyone, I guess; just read the author's disclaimer there in the front, notice the quote at the beginning under the title, pay attention during the first chapter or two, and don't say we didn't warn you.

(Also, don't read the Wikipedia article's spoilers. It will ruin your experience.)

The Road by Cormac McCarthy, if you're into utterly bleak post-apocalyptic fiction.

"La invención de Morel" ( Morel's Invention ^[1]) by Argentine author Adolfo Bioy Casares ^[2]. He wasn't a sci-fi writer, just fiction, but this novel is one of the best SF books of Spanish language.

The Inheritors ^[3] by William Golding (the author of Lord of the Flies)

Arthur Conan Doyle wrote a number of sci-fi stories. I've had the occasion to read The Maracot Deep. There were definitely some good parts to it, and it was reasonably creative and forward-looking for 1929 (though it turns out Doyle apparently hadn't picked up the implications of the Michelson-Morley experiment).

It's not the author's strongest work, but if you value old science-fiction, it's worth a read.

Joseph Conrad and Ford Madox Ford's The Inheritors: see here ^[1] for a little bit of info and a Gutenberg link.

Abduction ^[1] by Robin Cook ^[2]

If anybody's interested in short stories, Kipling's "As Easy As A.B.C" is at least interesting.

The first Century after Béatrice ^[1] by Amin Maalouf ^[2]. Amin Maalouf is a Lebanese author writing in French, well known in france for his historic novels happening in the Middle-East.

In this book, he supposes that, in the near future, a doctor find a cheap way to decide the gender of ones children, and explore the consequences on the world history over the next decades. It is (at least in French) a very well written Science-Fiction book in a rather grim near-future.

Also more "novelette" than "book", but:

Defending Elysium ^[1] by Brandon Sanderson (fantasy author).

Host by Stephanie Meyers, better known for her vampire fantasy books. The basic premise of the book was not particularly original, and I wouldn't particularly recommend it.

Edgar Rice Burroughs is mostly known for his Tarzan books. He also wrote a series about Mars (Barsoom) that has not held up well over time.