This is the earliest book by Arthur C. Clarke I've ever read. After hyping 2001 to my friend, he lent me a beaten copy of this book by the same author. I should mention that during junior and senior high, a favourite pastime of mine was to find all the collections of short science fiction and read all of them. Those who know anything about schools (Canadian at least), know that the books are out of date to the tune of the one on the dedication plague by the front doors. Nevertheless, these stories were the best that I've read. The lack of knowledge of the universe, and even the solar system, let some truly fascinating tales be told. This book is written in that same style.
Mankind is visited by an alien ship, and though the inhabitants are never seen, they claim to be there to guide humanity through a phase of life common to all intelligence in the universe. What follows is the struggle of a group of humans to figure out what's going on, and how the changes going on around them will affect humanity.
Though it's been a long time since I read this, and is a very old book, I still think it's good. Arthur C. Clarke appears to have been writing good sci-fi for longer than I thought.