Try ‘Uglies’ by Scott Westerfeld. It is the first in a series, but it stands alone more than the other books that follow it. It’s about a fifteen year old girl in a future where everyone is given an operation to make them beautiful once they turn 16. It’s a really interesting book and has great characters. There’s a bit of romance there too. It’s the kind of sci-fi that even non-sci-fi fans can enjoy.

Well a great Sci-Fi series is The Snow Queen by Joan D. Vinge
as far as horror goes…

pretty much anything by Stephen king
The Forbidden Zone by Whitley Strieber
The Exorcist by William Peter Blatty
Cabal by Clive Barker

How about ‘Stranger in a Strange Land’ or ‘The Left Hand of Darkness’? Both are weird love stories, and classic works of science fiction. Or if you want something shorter, but still a great story (no love story though) try ‘Enders Game’.