It's actually very important to ask people for recommendations for books, and especially for sci-fi, since it seems like a large majority of the work out there is well, garbage. Not to be too harsh, as IMO, the same thing could be said for a lot of artistic genres (anime, modern action film, etc, etc.).

For sci-fi, there are some really top notch work out there. But be warned, that in general the rest of the series isn't as good as the first book. Some classics, all favorites of mine are:

• Dune (Frank Herbert)
• Starship Troopers (Robert Heinlein)
• Ringworld (first book) (Larry Niven)
• Neuromancer (William Gibson) (Warning: last half of the book becomes s.l.o.w. though)
• Fire Upon the Deep (Vernor Vinge)