It’s also textbook hard SF - about an invasion by aliens who are only slightly more advanced (like, maybe a century) than we are. Spoilers:
The aliens arrive in a generation ship powered by a Bussard ramjet, achieve superiority over Earth by dropping objects on us from orbit, and in the end, humanity wins by implementing Project Orion - meaning, by launching a massive spacecraft powered by exploding atomic bombs and then firing every weapon we have left at them from orbit. It’s glorious.
Thank you for all the recommendations! That’s a lot of good recommendations to add to my list.
I have read many of the older novels recommended here (everything by Clarke, many by Forward, Asimov, Niven, etc). But if anyone else is interested, I can vouch for them all.
John C. Wright’s Golden Age trilogy is about life as a dissident in a golden age of humanity. The protagonist, however, sees himself as attempting to move humanity forward from stagnation, and while there is plenty of struggle and frustration, the general mood is consistently positive.
Without spoiling, I think I can describe it as the exploration of an alternative global reality, set in the early 20th century. It is dramatic, not humorous, but it’s not dystopic or depressing. The imagination and originality is superb, it’s a unique book. Although it’s not chock full of science like something like The Martian, it’s quite definitely in the sci-fi genre (it’s not fantasy), and I think it will appeal to somebody asking for hard SF, because ultimately it’s the alternative reality itself, and what underlies it, that’s at the core of it. It’s not just an incidental setting for the characters to play out their drama.
“Rocket Girls” by Hosuke Nojiri. Similar to ‘The Martian’ in that there’s a focus on the practicalities and logistics of space travel; generally very upbeat and has a happy ending.