A pop-science book, explaining how science-fictional concepts — for example, gravity control, or faster-than-light travel — could be achieved using existing science. In many cases the science needs to be helped along with heroic feats of engineering and/or politics — for example, when he's talking about space elevators and space fountains.
Most chapters are illustrated using one of Forward's SF short stories, or with an excerpt from one of his books. The ideas are interesting and they're explained effectively; I think his writing style works much better for non-fiction than for fiction.
ISBN 0671876864.