"Computer Lib/Dream Machines", TedNelson, 1974. ISBN 0893470023.
It's a "flip book". The first book is "Computer Lib", explaining computers to the layman in an entertaining style. Flip it over, and the second book is "Dream Machines", a (still only partly realized) vision of what computers could become.
It completely changed my view of computers at an early point of my life, including being the first time (as it was for many readers) I heard about hypertext, and about the XeroxParc DynaBook project with WimpInterface (graphical windows, mice, pointers), and the SmalltalkLanguage and its attempts to be simple enough for grade school children to use while still being very powerful.