The core of almost all hard science fiction is to start with a premise that is not currently, or ever, possible (what if we could travel faster than light? Fly at will? Visualize the Internet in our heads?), but to then keep the rest of scientific knowledge the same and speculate about what happens in a story.
By this definition, Randall Munroe's What If? is very close to science fiction. All it is lacking is the story. It should be required reading for anyone considering writing science fiction, since it's basically a book of premises. But I don't really think I would have to tell an SF writer that, I'm quite sure they have all read it by now.
If you are at all interested in science and are reading this, you have heard of xkcd. Munroe has been drawing this comic for many years, starting as a NASA engineer. He has made enough fame and fortune off this work to kick the NASA gig and write full time. In What If?, he takes his readers' most elaborate and interesting hypothetical questions and tries to answer them with as much accurate science as possible. He is thoroughly and amazingly successful at this--it's clever and interesting all the way through. It's written in short chapters so you can consume it in little bites, and this is actually a pretty good way to read it. I've been reading bits and pieces off and on for several months.
Munroe has a clean, clear, humorous writing style that will last forever. If he wrote a novel, it would read a lot like Andy Weir's The Martian, for similar reasons--they are both engineers that obsess over the facts and just relate them straight out. Maybe Munroe will try it sometime. I loved it and give it my most utterly rare 5 star review as required reading. If for some reason you have procrastinated, go get it now.