Eon
Greg Bear
** spoiler alert **
Greg Bear died 19-Nov-2022, almost exactly a year ago today. He was one of the best hard science fiction authors, and in my opinion, his The Way series was his best work, which would put it among the best of science fiction. He was active as a writer starting in the late 80s which would put him after the New Wave (Le Guin, Zelazny, Delany, ...), so that Bear had the advantage of flourishing in a time when science fiction was allowed and even expected to have literary merit -- you know, original plots, three-dimensional characters, with some women among them -- all that stuff.
The Way has all that, but for me personally (your feelings may well be different), the most interesting and exciting part of Bear's stories was the scientific backbone. Now, I want to be clear that, when I call Bear a "hard science fiction author", I don't mean that everything he wrote was based on established science. No -- there is a hefty dose of original speculation. It is always well-informed speculation and well thought out.
The story begins when Earth is on the cusp of a nuclear apocalypse. A new asteroid is discovered with a very peculiar orbit, and an even more peculiar resemblance to the known asteroid Juno. A team of scientists is sent up to study it. They discover that the interior has been hollowed out, and is full of equipment and even has people living in it. And there's a library that appears to recount the history of Earth, but this history includes events in the future of our team, and even predicts the war they wanted to avoid.
But it is in the seventh chamber that the big surprise is found. The seventh chamber is the start of a tube much larger than the asteroid it appears to be inside of -- in fact, as far as anyone can tell, it is infinite. This is The Way, which gives the series its name. The scientists build ships that can navigate the way. They discover that it has gateways that allow one to exit the Way. If you do this, you end up in a new universe. So we get to have alternative universe stories. One of the best of these occurs in the second novel Eternity, when Patricia, one of the scientists, is trapped in an alternative universe where she finds herself in a place that seems a lot like ancient Egypt. She rises to become a powerful priestess/executive, and never stops looking for a way back.
I read Eon in 1990 or thereabouts, I think. It blew me away. Subsequently I read everything by Greg Bear that I could get my hands on. Blood Music was particularly good, but The Way remained my favorite.