The End of Eternity by Isaac Asimov
This is the Asimov that tends to be both overlooked and under-rated, and it might be his best work, a tight paranoid little time travel puzzle box, brilliant in its construction of a civilization of time travelers. The premise: an organization called Eternity is a collection of agents, the Technicians, who exist outside of time, making the minimum necessary changes to ensure that humanity doesn't suffer greatly, preventing nuclear wars and the like, up and down the centuries, to produce a more peaceful, less chaotic human existence. And the cost, quietly, is that preventing war and hedonism leads to the loss of innovation and end of space flight, that nothing heroic happens either and humans stagnate, because every well-intentioned attempt to protect man from himself ends with the stars out of reach. You travel upwhen and downwhen through it, and despite using made-up words such as "upwhen" to mean "future," this book draws you right in and keeps you glued to the pages. Andrew Harlan is a Technician with whom the whole order breaks when he falls in love with Noys, a woman plucked out of time, which he is absolutely not supposed to do, since there are strict rules governing Eternal's interaction with people living in time, and to keep her he's willing to do anything, including destroy Eternity itself. What I admire is how Asimov confronts these paradoxes head on and provides extensive explanations, every clue there until, at the end, all the clues are pieced together, though the concept of messing with time gives me a headache, but I mostly stopped poking. The reveal about the Hidden Centuries, the ending blew my mind: those far up when centuries that are hidden even from Eternals, where the people in the Hidden Centuries have evolved even beyond the Eternals precisely because the formation of Eternity has stalled evolution, no more danger or stress to sharpen the species. His usual weakness is here and it stings more than the romance being stiff: Asimov doesn't include a single one where women become scientists and engineers and might join the Eternals' boys' club, so Noys is the only woman in the book, and to him the idea of women as anything but sex objects to be coveted just never occurred, a 1950s blind spot he never questions. But the structure is the art. An unputdownable Asimov thriller that is also a tragedy and an argument, what is more fundamentally important, freedom or happiness, one that warns of the consequences to humanity when eliminating unusuals and choosing mediocrity and safety over risk, that we need the challenges and the risks in order to develop, and to find life fulfilling, and it lands all three. This is actually quite a short read, perhaps mercifully so. If you only know him from Foundation and the robots, this is one of the best non-robot-empire-foundation books by Asimov. Pick it up you will not be disappointed.