1984 by George Orwell. All I can say is this: IF you have not yet read this Book then you should do so as soon as possible.
Fahrenheit 451 by Ray Bradbury. I do not think there is a single Book by Ray Bradbury that I would hesitate to recommend! Ray Bradbury is one of a few dozen World Famous Authors who I have met in person and who spent a significant bit of time with me, directly!
I, Robot by Isaac Asimov. For me it is hard to pick among Asimov's works; many are truly great reading. I like his short stories more so than his lengthy novels (Foundations Edge, etc.) . Pebble In The Sky is a great one too. I also recommend his non-fiction books too. He was the most prolific writer ever! In 1989 I had the opportunity to meet Asimov in person, one-on-one, and spend almost 50 minutes exchanging ideas with him! Little did I know that he was going to depart this World within a few short years. Fortunately our encounter was photographed. CLICK HERE.
Rendezvous With Rama by Arthur C. Clarke. One of the first Sci-Fi novels I ever read.
The Martian Chronicles by Ray Bradbury. In my opinion just as important as Fahrenheit 451.
Shike by Robert Shea. This is such a Powerful, Sensual and Exotic piece of Erotic Historical Fiction. A glimpse into Life in early Shogun-era Samurai and Ninja Japan. Very entertaining and somewhat educational!
NINJA by Eric Van Lustbader. Very intelligent writing.
MARS by Ben Bova. Aside from the fact that Ben Bova and I have shared a few thoughts via Cyberspace; I think his books are very interesting and fun to read.
Mother Night by Kurt Vonnegut Jr.. Vonnegut is another Author who I was fortunate to have met many years ago. I also read his "The Monkey House" .