An entertaining read, recommended for people who likes physics, yet does know that this book is 50 years old.
It proposed an easy way of traveling through space, by exploitation of black holes. The author treats black holes as something you can just create in a few years and then use commercially. Quite hilarious if it was possible! :) Of course, physics has moved on since then, so it is very unlikely to happen.
He does take some serious liberties when discussing his theory, such as when dealing with economics, but hey, it is fiction!