Many of the books I could recommend myself (e.g. Mythago Woods by Robert Holdstock) have already been mentioned here. So here's a look at some of the authors and books that aren't on the list.
Fantasy first, as lesser known authors and books in this genre have always been more likely to catch my attention than in science fiction.
So I'll start with Chosen of the Changeling by Gregory Keyes. This author has a degree in anthropology, so it's not surprising that this dylogy shows contact between several societies at once. Some of the characters belong to a society of primitive farmers, others to a nomadic society that looks like a cross between the Comanches and the Mongols, and a third society resembles a hydraulic civilisation of the kind that appeared in the major river basins of the world during the Bronze Age. And it's all very realistic. And there are also creatures that look like Neanderthals. But they're called Alves, which is almost like elves. Who else but an anthropology professor would think of Neanderthal elves?
For example, when one of the main characters, Perkar, arrives in the capital of the 'hydraulic' kingdom, the streets of the city seem like canyons to him, because he has seen a lot of mountains and canyons, but this is the first time he has been in a city.
And there's also a very interesting plot there. As I said, the Nhol Kingdom is very similar to a hydraulic civilisation, like ancient China or ancient Egypt. The main rivers - the Huang He or the Nile or some other river - were considered living beings and even gods in those ancient societies. So in this dylogy the river is really alive! But I won't go any further, as that would be a blatant spoiler.
This author also has book series called The Kingdoms of Thorn and Bone. It's also very well written and well worth reading.
The next author is Cecilia Dart-Thornton. Her fantasy trilogy The Bitterbynde is very well written and her writing style is very unusual and slightly reminiscent of Eddison's The Worm Ouroboros. For example, she uses a lot of old words and things like that.
In general, in this trilogy, the author has managed to bring together almost all of European mythology, starting with the story of the Pied Piper of Hammel and ending with the beautiful legend of Midir and Etain, and yet tie all of these different stories together almost seamlessly. I think that's quite an achievement!
Other books by this author, including a short story set in the same world as The Bitterbynde, are also very well written.
I would also recommend the Nightrunner series by Lynn Flewelling. The world building is excellent and the plots of most of the books are very interesting as they are more like crime stories or thrillers. They are always built around a mystery that the main characters are trying to solve. But her other book series, the Tamir Trilogy, is much worse, even though it takes place in the same world.
I would also recommend the Vlad Taltos series by Steven Brust.
As for standalone books, I would recommend Lud-in-the-mist by Hope Mirrlees. It is very poetic and beautifully written.
For science fiction I would recommend DarkWar by Glen Cook. There are canine witches-bitches flying around in space on wooden ships. I haven't seen anything like that from any other author!
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