WARNING: massive spoilers below.
If you want to introduce a friend to the world of hard science fiction, Footfall by Larry Niven and Jerry Pournelle would be a perfect book to hand them. The plot is easy to explain and common enough: aliens discover our solar system and find out planet to be habitable, attack, invade, are repelled, humanity wins, all is well. A million Hollywood movies have been built around exactly that theme.
But from the moment you see the book jacket, you find out it's not quite that simple. The aliens aren't slimy, or bony, or covered with sharp evil-looking bits or anything like that. They all resemble baby elephants with bifurcated trunks, and the sight of several of them parachuting into the American heartland actually makes the human observers laugh out loud.
Then you get reading and find out that there's no artificial gravity or death ray lasers on their spacecraft. No, it's been plodding through space for literally generations at sub-light speed, and took a short detour through the asteroid belt to pick up some weapons. No senseless risking of fighter spacecraft for these guys, oh no. Instead, they drop meteors from orbit on major cities and power stations. With incredible accuracy. Far less risk involved that way, you see.
These aliens aren't here to sap our oceans dry or impregnate our populace, or anything so cliched. They just want to live here, and humans happen to be in the way. So they go to war with the species, which from their cultural perspective is the only thing to do. They keep getting these transmitted requests for peace from us, and it boggles their minds. How can you make peace without a war first? And so the invasion continues.
But the kicker really is when humanity fights back, and this is where the "hard" science fiction becomes most interesting. Because the aliens are still in orbit, you see, and you can't just send up a space shuttle to try and take them out. So they build humanity's first orbital warship, using the best technology available at the end of the twentieth century. The cover up a building, make it look innocent and ordinary, then proceed to build a hemispherical metal shell with a cockpit on the top. Rails along the inside of the hemisphere are used to drop nuclear bombs, which explode at the center of the sphere and provide the fastest propulsion possible. (Sure, it also destroys the city for good, but this is an emergency.) For weaponry, you send self-targeting gamma ray lasers down rails on the outside of the hemisphere at the same time you trigger a nuke: the stray radiation powers the laser and destroys it at the same time, but not before it gets off one heck of a blast.
I myself finished this book days before going to see the movie Independence Day for the first time. It ruined the movie for the nitpicker in me, but made me appreciate the book all the more. After all, what's the point of science fiction if it doesn't use real science?