The Caves of Steel by Isaac Asimov
The first of the Robot novels (of which I, Robot is number 0.1).
Simply put, it's a murder mystery set a few thousand years in the future.
A more roundabout way of describing it would be to say that humans have colonised space and created artificial intelligence. On Earth this means everything from overpopulation, ever imminent starvation and extreme pollution to robots obviously robotic-looking taking over simpler duties in workplaces. Discord is growing. In Spacer colonies it means bureaucracy is king and empathy is dead - humans with any gene defect are euthanised as to avoid the troubles so rampant on Earth - and advanced humanoid robots are the latest secret project.
A Spacer is murdered on Earth and Detective Elijah Baley is called to the scene to solve the murder before the partner he's forced to take on does. I present to you: humanoid robot R. Daneel Olivaw, the latest of Spacer technology. In fact, so recently out of production that nearly no one knows he's a robot (indicated by the 'R' before his name which Baley conveniently leaves out whenever introducing him to anyone seeing as no one likes/trusts robots).
They set forth on a journey together not only solving the case but also a much deeper political plot.
This is classic scifi at its best! We're presented to big questions on existence, the fear of tomorrow, what it means to be human, and what makes a good society. It's philosophical and deep yet easy to take in without getting lost along the way or sacrificing the plot only to present some strange view on the human condition.
Caves of Steel is also obviously a child of its time and being first published in 1954 means that I had a hard time finding any female characters, even more of a hard time finding women with any sort of agency. There's Baley's wife, but she's not of much use to the story. After that the list of female characters abruptly ends and yes, of course that sad fact makes me grumble but I have to admit to preferring this kind of representation in classic scifi (which is barely any at all) instead of the kind I've found to be rampant in a lot of Heinlein's scifi where women are either vapid and useless or manipulative whores (Door into Summer (publ. 1957) and Time Enough For Love (publ. 1973), being the worst of what I've read so far, jeez... I couldn't even finish Time Enough it was so bad). Then on the other hand, I expect so much more out of anything written after the 1980's (thank the gods for Carl Sagan) than I do out of what came before (e.g. the Robot novels). I desperately need people of my generation to be better than their predecessors and write better (ie MORE INCLUSIVE) fiction but that does not mean that I'll excuse bad representation in classic scifi.