My favourite sci-fi book by far is Olaf Stapledon's '
Starmaker' (1937) followed by his work '
Last and First Men'
(1930).
Starmaker Wiki:
en.wikipedia.org...
Last and First Men Wiki:
en.wikipedia.org...
Both works were very much ahead of their times. The science in both still holds up today, and the whole time I was reading
Starmaker, I
couldn't get over the fact that it was written in the 30's.
From Wiki's description of
Starmaker:

Some of Stapledon's ideas for alien minds, such as collective intelligence, seem far ahead of their time, anticipating recent ideas about
swarm intelligence and the general fascination with networks. He also mentions the idea of virtual reality in the first alien world visited, in the
form of an apparatus that directly affects sense centers in the brain. The idea of entire worlds as spacecraft is used several times.
...
The travellers encounter many ideas that are interesting from both science-fictional and philosophical points of view. These include the first known
instance of what is now called the Dyson sphere, reference to a scenario closely predicting the later zoo hypothesis or Star Trek's Prime
Directive[1], many imaginative descriptions of species, civilizations and methods of warfare, and the idea that the stars and even the pre-galactic
nebulae are intelligent beings, operating on vast time scales. A key idea is the formation of collective minds from many telepathically linked
individuals, on the level of planets, galaxies, and eventually the cosmos itself.

I loved his method of story telling -
Starmaker was from the 1st person point of view - the narrator through some miraculous means is able to
travel the universe and the expanse of time in some sort of OBE. He describes what he observes during his journey, and eventually he finds himself
with the intelligence called 'Starmaker' - a being like God.
These books are often pointed out as being the best sci-fi ever written amongst sci-fi authors, however I rarely hear them spoken of in the
mainstream. I just wanted to put this out there because they're both worth reading, and
Starmaker is absolutely incredible.
Also '
Rendezvous With Rama' by Arthur C. Clarke is my 3rd favourite sci-fi book, but Clarke is hardly forgotten nowadays...
Lastly, I'd just like to add I've written a short story and posted it on ATS entitled
These Dark Worlds which is the prologue to a novel I'm writing - the story is
inspired by portions of
Starmaker.