Clarke's best. This is the hard sci-fi book by which all the others are measured. A group of people explore an alien spacecraft. Clarke shows us what they find, but makes no attempt to explain its purpose. He lets it stand completely on its own, and it's one of the most fascinating trips I've ever taken. 
 It makes no secret of being about the environment and technology instead of the people, and it works. Putting character sub-plots in here would've been distracting from this fascinating spacecraft. Who needs character development when you have a wonderful alien vessel to explore? 
 There is no real story. It's about a crew of humans exploring an alien spacecraft, trying to figure out who the aliens are, why they built this vessel, and why it entered our solar system. It's a great mystery, and one of those times when the questions are more interesting than the answers.