This 1979 book was the first of the Gaean trilogy, the others being Wizard and Demon. I have named the trilogy as among my top 20 favourite SFF stories (they feature the same central character in the same location throughout, so really make one more or less continuous tale) but it's been decades since I last read them, so I was pleased when Titan was chosen as December's read for the Modern SF discussion forum.
The story begins in a not-too-far distant future when the first manned voyage is being made to Saturn, under the captaincy of female pilot Cirocco Jones. As they approach the planet they discover an unknown satellite, which turns out to be a solid wheel-like artificial construction 1,300 kilometres across. It is mostly matt black, which accounts for the failure to spot it before. This captures their ship, and the crew go through a period of unconsciousness before being ejected onto the inner surface, into a habitable world which they dub Gaea.
So far it sounds like Clarke's Rendezvous with Rama, but as the story develops it becomes a lot closer to Niven's Ringworld, only even stranger (both books have been previously reviewed on this blog - see the list on the left for live links - and both also feature in my top 20 favourites list).
The crew of seven have been changed during their period of unconsciousness, acquiring different skills and attitudes which equip them to cope with the wide variety of environments and beings which they encounter, some of them decidedly bizarre. The last third of the book is taken up by an epic, months-long climb up one of the enormous cables which stretch from the inner surface of the rim up through huge spokes towards the hub of Gaea where they believe a controlling intelligence resides. There are some real surprises as they finally discover what Gaea is, and what has happened to them.
I earlier drew a comparison with Clarke's and Niven's books, but Varley's writing is much stronger on characterisation and relationships - not to mention sex. However, this does not detract from the drama and mystery of the story, which stands up very well as one of the modern classics of science fiction.