I was going to mention that. From a science fiction viewpoint, I'll add;
Frank Herbert -
Dune. The sequels - by Frank, not the
NuDune books - are optional.
Soul Catcher may be his best book besides
Dune, but it isn't sci-fi.
Arthur C. Clarke -
2001: A Space Odyssey,
Songs From Distant Earth and the
Rama books.
Isaac Asimov -
I, Robot and the
Foundation Trilogy. They're more designed for teenagers, but I still think you should give them a read. They're only around 200 pages each.
The Rest of the Robots is optional, as are the later
Foundation books.
Timothy Zahn -
The Thrawn Trilogy and
Outbound Flight.
The Hand of Thrawn Duology and
Survivor's Quest aren't must-reads in and of themselves, but should be read to improve your understanding of the others.
Allegiance is optional. For non-
Star Wars stuff of his, I'd go with his
Conquerors Trilogy.
Robert Heinlein -
The Moon is a Harsh Mistress. Same disclaimer as Asimov.
Orson Scott Card -
Ender's Game and
Speaker for the Dead. He went downhill after those.
Richard Matheson -
I Am Legend and
The Incredible Shrinking Man. Again, same disclaimer as Asimov, with the added proviso that subsequent events make these books look like jokes. But they are still good, and
I Am Legend pretty much invented the survival, scientific occult, and zombie apocalypse genres.
H.G. Wells -
The War of the Worlds,
First Men in the Moon and
The Time Machine. For the same reason as Matheson.
I'll probably think of more later. These are all things you should read if interested in science fiction, especially if just starting out - in which case you should go for the easier stuff on the list first. You don't want to pick up a Kevin J. Anderson book thinking it's sci-fi, after all.