I read a psychotic amount of books when I'm not at college, enough that I have to re-read books 3-4 times to save money.

Definitely pick up both "Ender's" series by Orson Scott Card. The 4 book Ender saga really delves into metaphysics in the later two books and gets pretty interesting. The Bean series, starting with Ender's Shadow is another great series, focusing more on international politics and war. Good strategy stuff and covers things like genetic manipulation.

Also on the sci-fi stint is Starship Troopers. If you've seen the movie, don't even think to associate it with this book. It's got tons of great military philosophy and is more about responsibility than anything else. It actually changed my political views completely.

Prey and State of Fear by Michael Crichton are also great books. State of Fear has gotten a lot of flak lately for it's stance on global warming, but that's really not the point of the book. The great moral of the story is summarized by a professor in one of the later chapters. Read it for yourself if you're curious. (A part of it is checking the facts yourself and making your own opinions.)

If you want more fantasy stuff, try some of the novels by Kelley Armstrong. The first book is about werewolves and is entitled Bitten. I really like her writing style and her books are among some of my favorites.

Along that same vein are the books by Kim Harrison. They're a little flakier than Kelley Armstrong's, but I liked them just the same.

If you want a really weird book, read House of Leaves by Mark Z. Danielewski. The story starts with a family who notices their house is larger on the inside than it is on the outside. It just goes crazy from there. The pages are written insanely (upside down, sideways, in strange margins and patterns). It's like reading nothing else.

Also, you might as well try some collected works of Nietzsche. Personally, I like the ones laid out in aphorisms and Thus Spoke Zarathustra.