Excellent Book! Why didn't I read this earlier? This book involves topics that have appeared before in many different SF stories, but I have rarely read a book that used the concept this well.
A interstellar craft appears in the solar system. A group of astronauts is sent to examine it whilst it is passing through. They find that it is a hollow metallic cylinder the inside surface of which has been terraformed and has an atmosphere capable of supporting human life. This book is one of Clarke's best, everyone should read this book.