This book is one of the definitive early books on Nanotechnology, from K. Eric Drexler, who developed most of the concept. The nanotechnology proposed by Drexler is not merely the sort of "nanotechnology" described routinely in the media in stories about new drugs or hair products but a fully capable system of "digital matter processing" using machines built to nanometere level precision, capable of self-replicating and doing for the handling of matter what computers did for the handling of information.
This book aims for readability and persuasion more than depth, but Drexler's other definitive book, Nanosystems, discusses the concept in more technical detail. For most readers, however, this book is an ideal introduction to Drexler's ideas.
If Drexler is right then once such technology appears the world will be radically different. Engines of Creation is an essential book for anyone interested in future technology.