It's been years since I read most of ACC's books but Rendezvous with Rama (the original 1972 novel) is one of his best. The follow ups were clearly written by a co-author with a different more conversational style I personally couldn't get into.
Another favourite is the short novel called A Meeting with Medusa, about the first exploration of Jupiter's atmosphere by a lone astronout Howard Falcon, in an airship, the Kon Tiki. Like all ACC's stories its based on scientific knowledge at the time.
The Fountains of Paradise (1977) is set in Sri Lanka and is about building a 'space elevator', literally a cable lift up to a satellite in orbit. Such a 'space elevator' would make tradtional rockets redundant. You still hear of this idea in science news nowadays as being theoretically possible when a strong enough material exists to be used for the cable.
A Fall of Moondust is one of his early stories, a kind of 'disaster novel' about a passenger vehicle on the moon that sinks in deep moondust. He must have written this story in the 1950s when scientists thought the lunar seas were deep areas of dust that wouldn't support the weight of a spacecraft.
Don't forget the short story books also. There are about eight and include Expedition to Earth that contains the short story The Sentinel, that '2001' was based on. Also The Wind from the Sun.
